keep in mind you are severely limited to other axlebacks you can run once you do this. Nobody tells you this, but I will. There are only a few quad tip systems out there if you ever get tired of the GT500 mufflers.....
Alternatively you can get the 11/12 single tip GT500 mufflers and those sound great too with no modding needed. They are like $330 brand new from LMR. Just thought I would put that out there.
I had thought about that quite a bit before doing this and realized the same rear bumper modifications are made for the Boss/CS rear valance. You technically could go back with a boss/cs valance and single tip axleback.