Motor size

Depends on how much money you have.

With a welder anything is possible. If you want AC and PS/PB and all those other goodies you can only fit a small block V8. But if you eliminate AC/PS/PB/ETC and clean up the engine compartment and run a small cowl induction hood you can fit a big block no problem. I'd look for a LSX 6.0 truck motor from a local scrap yard. Those with a big cam can make over 400HP and will gladly meet your emissions standards since it's a 99 and they've been in full size chevy pickups from 2001 to the current models so you shouldn't have many problems.

You can out up to a 454cid(8.1 vortech), but i would put in a gen 3 5.3 out of a C/K truck. You can have morehorsepower and better mileage that a 4.3.

I have personally seen a 502 crammed into one. I think they moved the firewall a little but with most of the newer motors with all of the electronics and the intake system i think you have to. I helped drop an LS1 into a '97 S-10 and the wall had to be adjusted just slightly.

a v-6

i am in the process of doing a LS1 swap in my s10...no cutting or welding has been needed yet !...it depends on the motor mounts that you used as too why u had to beat on the firewall, cuz mine goes right in no problems

Depends on the truck bed size.