D-Tape,
A 302 should drop into your 39. You probably know, but GMC made a 302 straight 6, so when you start talking 302...some folks think your talking a 6. I love the 302 V8...along with a 5 speed and a "modern" rear end, you should have quite the truck. If I were you, I would keep the 39 frame and mount the V-8 in it. Why? Because I don't have the patience to fit the cab mounts and everything else on an S-10. New spring perches on a modern rear end has the rear taken care of. There are V-8 engine mount kits around for reasonable if you want to keep the front leaf springs.
With all of that HP, I would change out the brakes as well and go to discs...A mustang II front end might serve you well too. There is lots of info on here ref. front end swaps...but as with everything regarding these old trucks...it's all in what you want to spend and how much work you want to do.
