Ummmmm....Borla is the quietest I know of, but they tend to get pretty loud with out cats. You could always try welding in a Magnaflow resonator before each of your current mufflers. They are pretty much "straight through" performance and are pretty good at taking the edge off loud exhaust systems. But I have never seen a Mustang with more than two mufflers, so I don't know what kind of adverse effects this would have on performance.