Just wondering if a 2.5 collector and 2.5 cat would be bottle necked going through a 60mm catback and would it even bolt up? I know 60mm is close to 2.5 inch, but is this what everyone uses when they say they have a 2.5 inch catback? I think 63.1mm actually equates into 2.5 inches. Just want to make sure I order the right header, cat, catback set up. Thanks.