What would an engine rebuild cost, or would it be any better to buy a used motor? If so, what would that run? I'd want to stay with the supercharged.

Unlkess you know the person you are buying the motor from I would go with the rebuild route.

rebuilding a GM pushrod is easy and relitivly cheap.

Sometimes, if you go to a reputable shop(by referral) and talk to them they can give you a window ($$$) to what the machine work will be. The cost will also depend on if you pull the motor, or pay someone to do it. The last V-6 (plain jane 4.3) I did ran about $1400 (1-1/2 year ago), that was everything including ...regrind roller cam, new pistons, heads and assembling rods on new pistons, rings, bearings, oil pump,lifters, gasket set, and I assembled. Usually they will assemble for an additional $150, but that will depend on the shop. As added insurance, I usually buy a new radiator and hoses. If you think of it, it is a cheap insurance policy. So it can add up.

its not thayt hard