My plans are to install an O/R X pipe but I still want a little bit more noise than the stock boss mufflers. My buddy gave me a pair of his GT 500 mufflers off of an 07.
My question is will they work/fit?

 

No,
there are differant hangars on the 2012/2013 Mustangs and the pipe diameter is larger on the 2012/2013.

 

Would suggest you keep the stock mufflers on until after you install the O/R X pipe.
It will be quite a bit louder with the cats removed.

 

Besides, the stock 07-09 GT500 mufflers are the same as the 07-09 A5 GT mufflers.

My suggestion;

http://www.dynomax.com/exhaust_detai...mufflers-39505

Pretty sur eit will work with the Boss although you may want to check with Dynomax.

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