Magnaflow universal Highflow 94100 unit with 3" inlet/outlet. Wish they made a 3.5" or 4" unit.
http://www.car-sound.com/02product/universal/941.asp
FYI, running a cat without the air pump is a good way of killing it in a hurry. I got enough overlap on the intake/exhaust that I still get cool flames during down shifts, which doesn't do the cat any good.
Wish we could run the race cat and be legal.
http://www.car-sound.com/02product/universal/race.asp