I have a '98 ITR, it has a JDM OEM 4-1 header on it, with a test pipe and stock exhaust from the test pipe back. Today I took off the test pipe and tried to put on the custom cat I had bought in 2001, it is a carsound cat modified by IPS Racing. The problem is that the cat is too short, the test pipe is about 17.25" long and the cat is about 16 and 11/16" long.

The only obvious solution I can think of is the donut that is placed between the inlet of the cat and the outlet of the header could fix the problem. The one in there has been crushed as it was installed with the test pipe, but perhaps a new uncrushed one would be big enough to make up the gap.

So ideas and advice are appreciated, I've emailed IPS as well.