Stock cats any difference from 2002 vs 2003
 

Well in order to pass mass emissions I need a stock h pipe as my catted MRT won't work. My question is will a 2002 catted factory h pipe fit on a 2003?

yes. as long as you are comparing gt to gt.

I live in mass as well and have had no problems passin emissions with my MRT catted h pipe. Why won't yours pass?

I'm pretty sure 99-04 exhaust systems for the GT were identical throughout the years.