Welcome to BonnevilleClub
Your engine is a Series I L67.
The aftermarket is pretty slim for parts for the Series I, so most people stick with a CAI, Hi-flow cat, ported exhaust manifolds, smaller pulley.... a muffler wouldn't really give any gains untill more modding is done, but it sounds a lot better though
preferred brands here seem to be Magnaflow, and Flowmaster.
A scantool will definatly help with modding.
Before you go with a smaller pulley, make sure you don't have any KR.
relax, ALL of your questions will soon be answered