What GM did was use the basic 3/4 ton chassis, but with 1/2 ton springs and axles. If the truck was spec'd to have the heavy duty spring option, then they got the base 8 leaf 3/4 ton rear springs, too. Actually, a longer wheelbase should ride better than a short bed, not as choppy.
For that handling you want, you might take a look at some of the IFS systems out there, and perhaps converting the rear to a 4 link. There a few companies making complete frames that are alredy setup that way, too.
If you go with the stock springs, you might take a few leaves out to lower it and reduce the spring rate (better ride) add a good set of gas shocks and front and rear anti-sway bars. I've done that to mine (exept for the leaf removal, I prefer the stock hight) and it handles very well. I have no weight in the back, but you could use sand bags, placed either directly over the axle, or better yet, behind the axle for better traction. I will warn you, these trucks do have handling quirks. especially in the rain. The best way I can describe it is to use a Nascar analogy, Tight in, Loose off.
For tha trans, since you're considering a big block, perhaps an M21/22 from a GM muscle car, or perhaps maybe a World Class T5. You'll need somthing that'll handle the torque. If you're not worried about the $$$, check out G-Force. They manufacture 4 and 5 speeds for Nascar and NHRA racers, they'll handle just about anything you can hook up to them.
