they are both 2 1/4", but the diameter is not the problem. the 2 bolt flange that attaches the header to the cat are off center. you have to either cut and reweld the flange on at the correct angle, or get an omni/blox adjustable test pipe. i read somewhere recently that the 90-91 teg cats have floating flanges that allow you to turn them whichever way you need them, but i haven't been able to verify this personally.