Welcome to the site. That's a good pick, going with the V6. They are a lot of fun. If you are concerned about gas mileage, I am warning you that it is tough to keep your foot off of the loud pedal for the first month or so.
As far as the exhaust goes, I can't be much help with the GT takeoff muffler. However, if you aren't dead set on that muffler, I would say give flowmaster a look. I installed mine myself, meaning anyone could do the same. I don't know if you want single or dual, but I believe flowmaster offers both. Just remember that if you go with dual exhaust, you need to cut the bumper to make room for the muffler on the left side.
