The American Muscle system (PN 17334) comes with the shorter Series 50 Delta Flow muffler, perfect for a frequent-driver classic Mustang. These are 19 inches long with 2 ¼-inch offset in and center out. This is where it gets tricky because '65-'66 Mustang factory dual exhausts are center in and offset out. With the Flowmaster system, there are no worries about offset. If you are fitting them to a factory '65-'66 exhaust system, you may need a muffler shop to tweak your pipes for better fit.