They both use the Kelsey-Hayes disc brake system, A bodies with a 4" bolt circle and Mustang with 4.5". It's just a matter of adapting the 4.5" rotor to your spindle, all the rest of the hardware is the same. If you want to spend a ton more money on aftermarket "kits" (some of which use GM parts, too), go ahead. The Kelsey-Hayes are arguably a better setup than even the later ('73 and up) A body system, which is the most popular route to a 4.5" pattern.
FYI, changing the rear to 4.5" involves non-factory aftermarket axles, so you need to get over it. None of the stuff comes from the Ford parts counter.
