[Mike C]

Looks a LOT lower in the rear than the '71 GMC that's sitting in my driveway. Are you staying 6 lug and going to run Blazer/4x4 rims or converting to disc brakes and 5 lug?

I'm a HUGE proponent for discs if you drive the truck at all in traffic. So, a couple of options. Try and keep a 28" tall tire all the way around or at least 27" in front (225 70 15 meets that) and 28" rear (275 60 15 meets that).

Switching to discs I'd run the drop spindles and stock springs with 5" rear spring then adding a block if you weren't happy with the drop. Or 2" front springs if you stay drum then a 4" rear. Add the block if you need more drop.