Before you do anything climb under your car a take a look at your existing cat. It has a unique to Porsche flange at both ends. To modify the new cat would mean cutting up your old cat or getting another stock one to cutup & weld the ends on the new one. Either way is expensive. The other alternative is use what is called a “test pipe”, about $125 from almost anyone. Most are considerable lighter gage steel than stock & all are federally prohibited on street cars. We’re went through 2 on the SC, "which is never driven on the street
", because the 1st one rusted out.
One interesting note is that a lot of ’84 to ’89 Carrera’s can pass emissions w/out a cat (must be in good mechanical condition & tune, fresh oil, air filter, full operating temperature). The point is if you have a Carrera, "which is never driven on the street
", & it can pass w/o a cat go right to SSIs or equal w/a 2 in 2 out muffler. Nice sound, a lot of +HP, less weight out back. Other wise get a free flowing muffler, the constriction in the stock exhaust happens well before the cat.