Well, you could put just about any GM SB or BB in that beast. I see a lot of 454 trucks being parted due to gas $$$ so my guess is you could pick up a fully dressed 454 for a fairly good price. From there you could rebuild it, swap it for a GM crate, a Jasper, or whatever fits your budget. Heck, you could even re-seal it an run it. In stock form the 454 wasn't a real performer but it did have a really nice torque curve which is whats gonna get that 5500lb truck rolling down the road.
If you wanna get a little silly you could even swap a 6.0 or 8.1 LS block in there but it's gonna cost you some $$ (possibly less than that GM crate though). I know there are carb setups for the 6.0 but you will need a stand alone spark controller. The 8.1 is nice but you'll need to run injection.
I recently picked up an '88 C30 with an injected 454 w/80k on it. I got the whole truck for $700. I plan to swap that motor into my K2500 for fun. Being that it runs good I'll probably just re-seal it an run it till it pops. Nice thing about the later 454's is the serpentine setups and better seals.
You have a lot of options, really depends what you are looking to do with it and what your budget is.
