Long and short bed rear bumper mounting differences

[s55mercury66]

Hello all, I am wanting to put a stock style rear bumper on a 1970 long bed GMC 4wd. The kit I have found is listed for a short bed. Has anyone here done this before, and if so, what did it take to make it work?

[Eagle9502]

From what I've seen there are two styles of rear bumper brackets for trucks:

1. Fits: 4x4 lwb, 2wd lwb coil or leaf spring, 2wd coil spring

2. Fits: 4x4 swb, 2wd swb leaf spring

[54blackhornet]

Why would GM make the end of the frame different between long and short ?

[s55mercury66]

The biggest difference I can see is the distance from the rear leaf spring hanger on the 4wd pickups, the distance between the rails is the same. I will have to take a look at the rails, but I'm pretty certain they are the same dimensionally. It appears it will require drilling holes to make it work, but without an original short bed rear bumper, I can't say for certain.

[Eagle9502]

It's the distance between the end of rear frame rail and the rearmost leaf spring hanger. If you look at the website of any of the parts vendors that sell them you can see pics of the two different styles. The Truck Shop, GMCPauls, etc.

[71CHEVYSHORTBED402]

I can say nobody reproduces the brackets for a 71 2WD, SWB with leaf springs. Perhaps there were variances in frame rails, by year.

Here's what I've found part wise, assuming the OP means a rear chrome bumper.

3890391-2 - Inner Brackets. 71-72 Ser. 10-30 (Stepside, Fleetside) and more.
Reads excludes leaf springs. Found an NOS on the Chevy site, it’s the bracket folks call “coil spring” inner brackets. The same part was used for the front bumper on 67-70 "pick-ups" (excludes 4x4)

3952089-90 - Inner Brackets. 69-72 Ser. 10-20 (4WD) (Pick-up)
Reads Rr. Bumper, which is rear bumper. No NOS available for reference. These are likely what folks call “4x4” inner brackets, it's the longer of the two common inner brackets.

3891985-86 - Inner Brackets. 68-72 SWB 2WD with leaf springs.
No reference found on the web for this bracket. Not surprising. My Chevy is an outcast

3891987-88 - Outer Brackets. 68-72 SWB 2WD with leaf springs.
My outer brackets are an oddity as well........... There are quite a few hits on the web for this outer bracket, reproductions. They're referencing the wrong part number. Been 50 years now, stuff gets lost. The brackets they're selling fit everything but a 71 SWB 2WD with leaf springs, I can't speak for other years.

3892477-78 - Outer Brackets. 71-72 and more. Ser. 10-30 (Conv., 4WD), Blazer (Stepside, Fleetside) and more.
Reads “Aux. leaf springs excluded”. Can’t say what an “Aux.” leaf spring is, but OK.
This is the common outer bracket. Found an NOS on the Chevy site, looks like the typical outer bracket. Other than that nothing found on the web.

On a side, the "coil spring" inner bracket is a 5" bolt pattern. I believe the "4x4" inner bracket is 7.2/10"ish. By the book your 70 seems to take the 3952089-90 & 3892477-78 versions. Check your bolt pattern to be sure. The outer brackets are the same either way.

[s55mercury66]

Thanks everyone for the replies. I have 2 swb 4wd trucks here to measure from, and it looks like the difference is definitely in the spring hanger location, relative to the end of the frame. The lwb has a 7" +/- bolt pattern, while the swb has a 5". I think we're gonna go for it, and see what happens. I'll report back with the results in a week or two, thanks again for the help.

[Eagle9502]

GMCPauls has both style rear brackets listed with pics on their website.

https://gmcpauls.com/bumpers1.htm

[s55mercury66]

Okay, I finally got around to mounting the rear bumper, and all it took was a little opening up of the rearward mounting holes in the frame, the hole indicated by the tape. You may have to open up the holes a little from front to back, if one side needs to go forward or backward. I bolted the brackets on by the forward holes, and scribed a line where I wanted to open up the holes. For those who want to mount a Blazer style rear bumper, it is a walk in the park, parts were sourced from LMC in this instance.

[s55mercury66]

Originally Posted by Eagle9502

GMCPauls has both style rear brackets listed with pics on their website.

https://gmcpauls.com/bumpers1.htm

Sorry to take so long, but thanks for the link.

[Eagle9502]

They've got good parts and support this site.