3 We’re fitting Gil’s GT with the 2-1/2-inch Flowmaster Force II system (#FLO-17106) from Summit Racing Equipment. This system practically hangs itself using your Mustang’s stock rubber hangers. Because it is a 2-1/2-inch system, clearances are tight, which means you must pay close attention to distances between pipes, mufflers, and floorpan. This is the Flowmaster Series 50 Delta Flow muffler, which reduces noise and resonance while improving the Flowmaster’s legendary throaty sound. You can get into this cat-back package for $399.95.