I had to change my gasket before the cat cause it was leaking and when I removed it I noticed that there was a exhaust stain on it like the gasket was off center (will post a pic later if anyone cares). so I pulled down the pipe and it was on correct but the opening was not bigger enough but it measured the correct size 2.5 gasket, 2.5 exhaust. so I pulled out the dermel and when to town and did alittle gasket matching.

exhuast leaked fixed and it should but alittle less restrictive
the lesson here is check your gaskets

 

I ordered my cat from Cattman. The one he sells has been dynoed proven to increase HP. Anyways, he includes Nissan Gaskets.

Otherwise, I'm waiting to see your pics cuz I can't figure out what you're trying to say.

Jae

 

what I am saying is put the gasket up to the cat and see if the gasket is the correct size no matter what the label says, cause nissan or not you may have to trim it alittle