The problem is a Mustang or at least most I know of, dont have the motor to sound good open header. In order to have a mean sounding car with open headers, you need more compression, a larger cam and such. A stock motor with shorties will sound horrible, even a decently built motor wont sound very good with open headers. The majority of cars that have them and sound good have race motors, which help the sound a LOT!
Id say just put on a good exhaust and it will sound good, draw less attention and make more power!