90-91s have a longer cat, therefore the b-pipe is shorter
92-93 have a shorter cat, therefore the b-pipe is longer

What usually happens when you buy a catback for either of these cars, is a 92-93 will come with an extra extension piece for the b-pipe, where as the 90-91 don't.

You can also easily just change the cat to the length you need (good time to upgrade to a highflow). Other than the length of the b-pipe, the hangers and bends are exactly the same.

(PS: Hopefully I didn't get this backwards )