dude just go to mufflertech.com and pay $100 for a carsound cat thats a direct bolt-in 2.5". The Donut gasket will seal the 1/4" difference (keep in mind thats 1/8" on each side)

If you really wanna make a fuss about that extra 1/8" all around, you can ask them to custom do one for you, but I had a 2" header and a 2.5" test pipe and the donut gasket sealed fine, I never had an exhaust leak.