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Old 03-14-2019, 06:35 PM   #1
Magicalzoomer
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1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Hi Guys,

First of all, sorry in advance if these are really novice questions, but any advice will go a really long way for me.

I am planning on swapping my 283 and 4speed manual for fresher 350 and TH350, mostly because the current setup will not allow me to go more than about 55MPH without revving too high and I believe there is currently a rear main leak, so the transmission will have to come out anyway. This will be my first ever engine swap and I was hoping to learn from your expertise.

FIT:
First of all, I attached pics of the motor and tranny that I am putting in 350motor/TH350 2wd Tranny. What should I check for to make sure that these will:
1. Match Together?
2. Fit into the C20?
3.Require Driveshaft to be cut?

PARTS NEEDED:
After some Research Here is what I am Figuring the parts I will need. Please feel free to add.
Items Needed
1. Transmission Crossmember
https://www.performanceonline.com/19...n-Crossmember/

2.Floor shift Kit
https://www.speedwaymotors.com/Lokar...xoCEmkQAvD_BwE

3. Any of parts Needed or Helpful?

ADDITIONAL GUIDANCE AND INFO:
4. Any tips and tricks to make this go smoothly?

Really thank you a ton for your help with this. This truck is my first build and I want to work to make sure that it is worthy. I really appreciate any guidance for a beginner like me.
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Old 03-14-2019, 11:38 PM   #2
Captainfab
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Changing to a TH350 will not lower your RPM's at all.
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Old 03-14-2019, 11:44 PM   #3
69 C10 LWB
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

I can’t speak to this specific swap because I’ve never done it but I know a few things you will need to do.
1. You say you have a 4 spd now, if it’s the truck trans you will have to remove the trans crossmember and get a new one to fit the auto trans.
2. With an auto you will need a trans cooler.
3. You will have to cut the driveshaft and find a slip yoke for your trans. The 4 spd driveshaft bolts to the trans while the auto has a slip yoke.
4. The engine will bolt in no problem and that trans will bolt to the engine.
5. Your top speed won’t change. You are swapping to another direct drive trans. If you want to lower cruise rpm you need overdrive or different rear gears.

Last edited by 69 C10 LWB; 03-14-2019 at 11:46 PM. Reason: Spelling error
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Old 03-14-2019, 11:50 PM   #4
69 C10 LWB
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

You will also need a new starter. The 4 spd starter bolts to the bellhousing and you will need one that bolts to the block- easy to find but another expense.
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Old 03-15-2019, 12:13 AM   #5
LostMy65
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Is a Th350 strong enough for a C20?
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Old 03-15-2019, 07:24 AM   #6
69 C10 LWB
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

It depends on how much power the engine makes. If itís just an average stock or mild small block it will be fine.
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Old 03-16-2019, 08:04 AM   #7
vince1
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

I am contemplating the same but going from 6 cyl to 8. I think the donor was a 70's Chevelle and it has a worn cam lobe so will be starting off with an overhaul. I'll also be looking at dual exhaust and most likely 4 barrel manifold and carb, electronic ignition, starter, etc, etc. including transmission refresh. At this point I don't even know if it is a 350 and am suspecting 307. 305?

Will your intake and exhaust manifolds fit or are they also on the replacement engine?
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Old 03-16-2019, 11:27 AM   #8
Magicalzoomer
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

69 C10 LWB
Thank you a ton for all of the info! This is going to go a long way for me.

1. Get a New Trans Crossmember
2. Transcooler-
3. Cut the Driveshaft and find a slipyoke that will fit a TH350 Trans.
4.New Starter

350/4speed Manual Question-
Out of curiosity- Would that 350 also bolt right up to the 4speed manual that is currently in the truck? How tough would it be to put in a bigger rear gear in the manual? I can just see the manual being a lot more fun than auto.

FYI-I will likely be putting in a light cam upgrade, Vortec Headers, and some type of intake manifold if that matters. Not crazy power though, just for a bit more fun if that matters.
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Old 03-16-2019, 11:33 AM   #9
jocko
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

The 350 should bolt directly to the current bell housing that is attached to the front of your 4-speed with no problems.
Swapping rear gears can be a bit of a pain, lash must be set up correctly to avoid whining, etc. Your truck either came with 4.11 or 4.57. Since 65s don't have SPIDs, there's probably no way to know without counting teeth or looking for an intact ID tag. Doesn't matter, both provide sky high highway rpm. If you aren't going to be driving 75 on the highway, then a 3.73 is a good gear for a mild 350 and will reduce your highway rpm somewhat. Some folks prefer swapping a hub to hub different rear to swapping out gears, but all depends what you're comfortable with. If not confident in a gear swap, I'd take it to a shop for that to ensure it's done right - or at least read up on it before ripping it apart to see what you're in for. One very nice option, if you prefer keeping a manual trans, is to swap a modern NV4500 trans (more appropriate than a 3500 for a 3/4 ton truck) which provides overdrive gearing for the highway and eliminates the somewhat useless granny gear unless you have a valid need for it. That way you can retain your 4.11 or 4.57 if you want and reduce highway rpm. Still, if it's a 4.57, it will still be pretty high rpm at highway speeds.... If I were you, I'd want to confirm what you have first - there are several methods, take a google on here about rotating the tires and driveshaft to count rotations - but it's not that accurate and it's easy to screw up, so please read up on it first. Best method is to count ring and pinion teeth - or look for a ratio stamped on those gears. Also look up NV4500 swaps on here, I believe several members have done it. If you happen to have a posi, I believe your gearing will be 4.57. If not, it could be either. What I DON'T know is if 4.11 was standard with the V8 that it came with - I don't think it was as 4.11 was the optional gear and 4.57 was the standard. In later years, opting for a bigger engine often brought with it lower numerical gearing because you didn't need as much torque multiplication to get the vehicle moving with the bigger engine. Someone on here can probably confirm whether a V8 got the 4.11 or if 4.57 was standard for all engines.

Depending on how much time you have to tinker and pocket depth ($), the easiest approach is probably to just swap the 350 in, keep the 4-speed. Button up that project first, then move on to the rear gearing. If you go the trans swap route, however, it makes the most sense to do it all at once while the engine/trans are out of the truck just one time.

Last edited by jocko; 03-16-2019 at 11:48 AM.
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Old 03-16-2019, 05:07 PM   #10
Magicalzoomer
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Jocko
I really appreciate the detailed information. I think those options are all really good calls. I’ll dive into this info and keep you updated on what I end up going with.hopefully I’ll get started this week. Thanks again!
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Old 03-16-2019, 08:57 PM   #11
66x3
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Hey Jocko,hope this is not considered a thread hijack.Someone let me know if it is but here goes. I just dragged home a 79 C20 frame today intending to use the front crossmember,power steering box, and possibly the rear end.Nearest we can figure is it has 3.43's in the semi-floater rear end. Do you guys think this would be too high geared coupled with my stock 283 and sm420?
Also considering with my Avion on the back weighing in at around 2300-2500#? Thinking it might be a weird combo with the stock 283.
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Old 03-16-2019, 10:17 PM   #12
jocko
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Quote:
Originally Posted by 66x3 View Post
Hey Jocko,hope this is not considered a thread hijack.Someone let me know if it is but here goes. I just dragged home a 79 C20 frame today intending to use the front crossmember, power steering box, and possibly the rear end.Nearest we can figure is it has 3.43's in the semi-floater rear end. Do you guys think this would be too high geared coupled with my stock 283 and sm420?
Also considering with my Avion on the back weighing in at around 2300-2500#? Thinking it might be a weird combo with the stock 283.
79 C20 standard axle ratio was 3.73 & 3.21/4.11 were optional. 3.40 was a C10/Big10 gear. 3.73 or 3.40 would work out quite nicely with your SM420, which has a fairly deep 2nd gear. Basically, assuming you're starting off in 2nd gear, it would be a little more peppy than a 700R4 with 3.73s. And your highway rpm would be reduced significantly as you can see in the chart below (assuming you're starting with either 4.11 or 4.57) (chart based on a 30" tire, but you get the idea). You're right to wonder if the stock 283 might struggle a little with 3.40 gearing and a camper load - but I think that the 3.57 2nd gear of the 420 will help you out significantly. If it's not enough, you can always use 1st
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Old 03-16-2019, 10:43 PM   #13
66x3
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Re: 1965 C20 SBC 350 and TH350 Swap Item List/ Questions

Thanks Jocko, thinking they must be 3.73 ,doesnt seem like anything higher would make sense in a 3/4 ton.I know a full floater is heavier duty but the semi floater is supposed to have been rebuilt recently and its what i have. Too bad, I like the looks of the full floater with the big hubs better!
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