There's a couple variables to consider when doing this.

What are your front brakes like? Discs or Drums? What years Mustang are you taking the rims from? 94-04 are different than the 05+ (current generation) because the newer ones have more offset. That will dictate a different width spacer. Some rotors have a different leading "snout" that allow you to use the rims as is while others may require that 1mm removed.



Those are Konig 17x8s on my challenger. They are for a 94-04 Mustang. They have a 28mm offset. I'm running 1 1/8" spacers (30mm). I am also using 11.75" rotors from Advance Auto. After the spacer is on the rotors, the leading edge of the rotor is just small enough to use these rims without cutting the center register (very happy when I found that out). Make sure you try your rims on BEFORE you mount tires. It should make life easier if you have to remove material from the center. Of course, the rear is not a problem because there's no register on the rear spacers and they jut out far enough over the nub that the rear rims would traditionally use.