On my 2011, the GT500 tips fit just fine with the stock rear valance. As mentioned, the back pipes of the mufflers will need to be cut off of of the GT500's, and the "donor" pipes of your 3.7 mufflers welded on to fit the stock pipes.
If your willing, search "exhaust", and my site name, and you'll find pictures of the welds that I had done. I posted this about a year ago.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.