The stockers were pretty much all 15"x7" with a 4.25" backspace, all should be the 5 on 4.5" pattern so they should all fit. There may be some 15x8's running around, but I think those were a slightly different style (later trucks).
I've bought a few original sets, and I'll say this- Summit sells reproduction police rims by wheel vintiques for around $100. They don't need to be cleaned and repainted, they should be straight and balanced, and you can get them in anything from 15x7 all the way up to 15x10 in a few different backspacings. After considering what I paid for the originals, then figuring on the time needed to clean up and paint them, its not much more (if any) to just buy new ones. And, you won't have to worry about them not being true, as I've found a couple originals that appeared straight but ended up being less than so.
