BBC/TH400 Conversion

[geunther]

I currently run the original 350/350 combo. I would not do anything to the truck that I could not return to original in a weekend. That being said, because I want to drive the truck and keep the original combo in the best shape possible, I am considering an engine swap.

I am considering mothballing the originals and swapping in a BBC and TH400 that I already have. I see that my frame has holes to move the crossmember back, presumably for the extra length of a th400.

My question is: With the tail shaft farther back, is the angle good for a one piece drive shaft through the factory riveted in crossmember (the one that hangs the carrier bearing)? This would allow me to remove the original drivetrain back to the differential without modification.

I did this sort of thing with a 70LT1 Corvette and sold it with both engines.

[sick472]

I looked into a one piece drive shaft when I decided to install the TH400 in my truck, but ended up having a new front shaft made for it instead. I also changed the carrier bearing to the more heavy duty one.

I think you would find that the angles are good, but clearance through the carrier bearing crossmember hole will be real tight, if it is even possible for you truck's stance. The last thing you need is to have to modify the carrier bearing crossmember by cutting the hole larger or entirely out on the bottom side.

[Palf70Step]

Is it a short bed or long bed? If it is short bed, I'd do the 1-piece drive shaft. Long bed it is pushing to do 1 piece. You'd probably be better staying with the 2-piece, just beef it up.

Never understood why some shortbeds came with 2-piece drive shaft

[geunther]

I thought of mentioning the bed length when I was posting, but Id forgotten in the 10 minutes it took me to type.

It is a short bed so hopefully a one piece will work out. With the tranny in place its an easy figure out, but I am just trying to account for every little alteration before I commit.

And I totally agree on not modifying the crossmember. I even thought about running the th350 for even less changes, but I already have the 400 and obviously it is a better choice for the bbc.

[kwmech]

Question should be is: what are you using for the engine mount frame stands?

[geunther]

Originally Posted by kwmech

Question should be is: what are you using for the engine mount frame stands?

I'll either use the 72 only stuff for BB (stands, clamshells, etc) or 71 stands modified for attachment to the 72 SB isolators that bolt to the block. I almost pulled the trigger on 72 BB stuff but still putting it all together before committing.

[kwmech]

Placing the engine on the big block stands in the forward holes will move the engine towards the firewall 1-1/4'', then you have the length of the t400. All the holes are in the frame even with this conversion. The frame pivot for the transmission linkage will need to be moved also.

[Pitpig]

Swapped in an LS with a TH400. No problems with single drive shaft or driveline angles. Got the driveshaft from Amazon of all places, around $250 with all u joints and slip yoke

[kwmech]

Bolt the stuff in, truck came with a big block and a 400 from the factory. I think you're over thinking it. Have a little faith grasshopper.

[geunther]

Originally Posted by kwmech

I think you're over thinking it.

I totally agree.

Thanks everyone. I think I have it all figured out now from your help and looking at factory BB/400s. I knew I would have to have a shorter DS and will measure that once everything is in. I think the only thing I need to source is the smaller linkage frame pivot and engine mounts.

[kwmech]

Originally Posted by geunther

I totally agree.

Thanks everyone. I think I have it all figured out now from you're help and looking at factory BB/400s. I knew I would have to have a shorter DS and will measure that once everything is in. I think the only thing I need to source is the smaller linkage frame pivot and engine mounts.

I may have the linkage piece, I'll take a look tomorrow--pretty sure I saw it last week

[geunther]

Originally Posted by kwmech

I may have the linkage piece, I'll take a look tomorrow--pretty sure I saw it last week

Thank you for checking.

[70STOVEBOLT]

I used all factory holes, but had to have my driveshaft shortened to 59.5”. Took out 250/3OTT and put in 402/TH400 using all factory big block components including the shift linkage, 68-71 mounts and frame stands, and one piece driveshaft. It has an HEI distributor and there is enough clearance to remove and install it with the engine in place. I don’t know if 2WD and 4WD frames are the same, mine is 2WD SWB.

[70STOVEBOLT]

That looks short, is it off a 4WD?

[kwmech]

Identical to the one on my 69 flatbed, who knows what it originally came off of