Knightmare69

Just wondering because I have a no name aftermarket header that I'm installing on my gf's 96 ex and having just measured the diameter of the pipe, I got a measurement of 2.78 for the outer and 1.78 for the inner. The outer is counting the gasket that goes over the pipe. Is this correct? Doesn't this mean that the stock converter has an inlet size of about 3in? I'm just trying to figure this out before I order my replacement cat. I was gonna order a 2.25 in and 2.25 out but with this new measurement I feel like I need to order a 3 in and 2.25 out. Unless I'm not suppose to run it with the gasket in which case I would be able to get the 2.25 in? TIA.