Need Help Identifying 4x4 Puzzle truck!

[jugadro]

OK,
So I have a Black 1970 K20 4X4 with a 402 BBC and a TH350 (I think).

I need to buy some stuff for the truck; U-joints, brakes, diff gaskets, etc. However, the VIN# decodes it as a 1970 2wd step-side, and it's making it difficult to get correct parts. I'm also trying to figure out what I have for axles, and eventually gears.
I did find a number stamped on the frame by the power steering pump - 0J125115.
I've tried looking for identification help but couldn't find much of anything.

Judging by illustrations in LMC catalogs, I'm wondering if it is sitting on a '73-87 frame? This is beginning to drive me .

I'll have photos of the underside soon, until then any knowledge would be greatly appreciated. Thanks.