You're in almost the same boat as I am. I love the way the SLP Loudmouth sounds, thus it's what I am planning on getting. What I have been looking at doing is modding out my entire exhaust...headers to tips. That's one route you can go. As I learned, Mac is the only maker of Long Tube headers and a H pipe for them for the V6 Mustang. If you look on this website under the V6 Exhaust there is another route you can go....
http://www.mustangtuning.com/v6duexadkit1.html That will allow you to adapt any GT exhaust you want onto your stock V6 system. I can't explain how it works exactly.
Hope that helps you out.