Yea, just remove the other nut (use some PB plaster or liquid wrench anti-rust) after thats done you can just remove the stock main cat and everything should be good (are you installing a test pipe in place of the main cat?)
The bolts you need are: 3/8" x 1 1/2" stainless steel bolts and locking nuts that can be found at any hardware store. Just use a washer on either end.
