[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.