I have MAC Long tubes and a pro chamber with Flowmaster 2 chamber cat back kit and stainless tailpipes. It sounds GREAT all around but drones a little (Flowmasters there). The mac stuff fits perfectly and doesnt leak anywhere. Same with flowmaster. If you want Mac, get it from
www.mustangwarehouse.com Thats where i got my headers ($220) and Pro chamber ($220) and they were both shipped free. I got them within 4 days of ordering (NJ to TN). Since you have a GT you can save a few bones on stainless tailpipes. LXs look horrible without them though. Get long tubes, either an X-pipe or a Pro Chamber, and it's up to you on the cat back. just make sure it's all 2.5 inch. If you can, ride with people that have the different brands of mufflers to see if you want to put up with the drone of the Flowmasters or say the quietness of Dynomax. I quite happy with my system, but I think soon I'm going to try some quieter mufflers. I plan to go on some trips this summer and I dont want 10 hrs of 2 chamber DRONE. Know'm sayin?