I have a 2000 Ej6 hatchback with a b18c1 with a skunk2 alpha header and a Invidia exhaust. Currently I have a test pipe that fit in between but I have to get a emissions test soon and would like to put a cat in there. What would kind and what would fit?

 

Take it to an exhaust shop, have them chop the flanges off the test pipe and weld them onto a proper high flow cat. Done. Problem solved, budget maintained.