How much is the cat back you're looking at? I also say go to a muffler shop and get a quote on a cat back system. I had my system made for my GT. X-pipe, super 10's and 4in tips. it was less then $400.
Its 380 from lastemodelrestoration.com. Its from the headers back all pipe. The muffler shop near me wans 550 for the system. I can put it on myself thats no prob, but I dont wanna buy somethin that wont fit