[75 911s]

Your car did not originally include catalytic converters. In 75 They introduced a rudementary emissions setup which included the Thermal reactors in California. I think 76 and 77 all US cars had them. Your 74 exhaust (if original) is actually very good. Unless you have money burning a hole in your pocket or the existing ones have been changed or rusted out, then keep them. Take them off, jet coat them and reinstall with new studs and hardware if you want. (studs are a whole different story - they break easily, so removing your setup can lead to a lot of pain and expense)

Start by posting a picture.

You want the throaty sound, convert your muffler to a 2 out (since it's already a 2 in via the 74 exhaust) or bolt on an MK 2 out. No need to change the desirable Heat exchangers out.

BTW in Arizona, pre-75 cars are exempt anyway, and of course you are outside of Tucson city limits so you are fine too. A cop can still cite you for being too loud though unlikely.