Well, SMS will make it to spec for you, so it uses a 21/4 inch back flange. Otherwise, just get a normal test pipe, and weld on a 21/2 inch flange to bolt up to the test pipe. It's not brain science, any muffler shop can do it. But you really are just better off buying one already done.

Look through an issue of sport compact car, and I'm sure you can find at least ten places that make them or sell them....

Later,
Aj