Unless a you know someone who is scrapping the Gt cat back, it would be more cost effective to go to an exhaust shop and have them do it. My dad just did it to his Titan and only cost $200 for the Flowmaster 40, dual 3 in back , and 3 in Tips.
Ask around town(your friends, not the shop) and find out which shops would rather not hassle with paperwork, there is your bargain!