You want the Camaro/Firebird/TransAm Crossflow muffler. Just like yours, those mufllers have an inlet and outlet on one end, and an outlet on the other end. It'* a near-direct replacement with much better flow. You need 2.5" inlets and outlets.
The rest of your exhaust is all 2.5" mandrel-bent stainless steel. Keep it. You'll lose low-end if you open it up. Keep the stock resonator, and upgrade your catalytic converter to a 2.5"x2.5" high-flow cat with rear O2 bung.
There are only 2 or 3 cars on this Forum that have pushed perfromance to the point that they need more drastic changes.