Stock size is 3.8, you could prob get away w/ running a 3.6 w/ a CAI/FWI, 180 t-stat, colder plugs and a tune, anything smaller than that would need supporting mods. Even w/ that you'll want to buy something to keep an eye on KR (like a LS1M Scanner or aeroforce scan gauge))
Better question to help you out is how fast do you want to go?
Right now you're prob @ about 15.0-14-8 in the 1/4.
If youre looking to drop pullies, i would buy an MPS (modular pulley system), trust me, once you start modding (and its hard to stop ) its easier to remove 5 screws to swap pullies than to remove and press on a new one