You'll want a muffler of this style (ask for one for a 1980'* GM F-body car). You won't an inlet any larger than a 2.5" or outlets larger than 2.5" because as they said above, you will lose too much torque -trust me mine has a 3" inlet and dual 2.5" outlets -I lost, and when I put the GM performance single in/single out on it I found I had gained back about 10-15 lb ft of torque (maybe more).
A view from the back of mine with the dual twin outlet tips.
A side view to get a better idea what they look like from "over there".
I used angled tips because our pre-1998 SE'* and SLE'* don't have exhaust cut-outs in the bumpers.
If you are going for the power and a sleeper look just have some stock turn down'* that hide behind the bumper -that'* what I wish I had done. That'* how they did it with the 1995 Bonneville SE'* that got the supercharger from the factory -not only is it sleeper, but it give'* the car an attitude andyou won't be ridiculed by the muscle car guy'* for being too gaudy with the exhaust (ricey although tips do tend to look alot better on Bonneville'* than on alot of other cars).