maybe you need a set of centering rings. im not sure if mustangs call for those but i know with all my other cars when i would buy aftermarket rims they would come with a set of centeing rings that center the rim on the hub. id look into that.
It might be that the rotors are warped. Or you might be needing an alignment and other suspension work done. Do you get chatter in the steering wheel when you hit the brakes?