Well, welcome to the BonnevilleClub
You came to the right place for questions and upgrades on your car
First off, make sure your air filter is shielded from the hot air under the hood, you want the coolest air you can get... trust me, it will make a world of difference
For the smaller pulley, you will want to obtain a scan tool and check for KR ( knock retard.... computer "hears" knock or predetonation through a knock sensor and retards timing ) There are many things that will cause knock, including cracked exhaust manifolds - very common on the Series I cars, regular fuel.... our cars are recommended for premium, and you really do need it
also, drawing in hot air from the engine bay will hurt performance as well.
For the muffler, you don't have two mufflers, nor do you need two. We have the same crossflow style muffler as the F-Body Camaro and FIrebird. The exhaust enters in one side, exhaust splits inside the muffler and is let out of both sides. our exhaust looks like this:
Also, don't forget a catalytic converter... you will lose low end without one, and a hi flow magnaflow can be had for $50 - $80
I would actually start off with a full tune up though... Plugs, wires, O2 sensor, PCV valve, fuel filter, clean / replace air filter etc..
There is TONS and TONS of information on here.... you should also get used to the Techinfo section at the top of every page. lots of good stuff in there