First off he's not in California so he's not subject to CARB rules.

The simplest way to get the header to fit the rest of the exhaust is to call up LandSpeed racing and order a cat that is designed to bolt into your car with whatever given header and exhaust you have. They make everything to fit.

Otherwise if you want to have the muffler shop do it, get a universal Carsound cat with 2.5" inlet and outlet and have the proper flanges welded on to it to mate up to the header and exhaust.