What I heard with their exhaust systems was that they rust, I also don't believe in duals that leave the Y pipe in there. Cut that thing out and go with true duals. As for Mac Mufflers, I had a set of their Stainless steel flowpaths and loved them. They sound great! I get the feeling you think putting a flowmaster muffler on your single exhaust system will improve your performance? Wrong, your stock muffler will flow better than the Flowmaster, the only thing a flowmaster does better than a stock Mustang muffler is make noise.