The NV4500 was used in a lot of full sized GM trucks and is a good choice for your application. Wikipedia catalogs a lot of good info on where you can look for one - New Venture Gear 4500 transmission - Wikipedia, the free encyclopedia
You might also ask the guys at Pacific Fabrication (where you were considering getting an engine/electronics package) if they can hook you up with a tranny, too?

The Tremec T5 is a lighter duty transmission, and you'd probably want to go through it and replace parts with beefier aftermarket pieces before trusting a used one in a truck, especially given that you're planning to drive "aggressive". I think I'd look for an NV4500, unless you want to save your pennies for a Tremec TKO 500 or 600....