The center register on Mustang wheels is about .039" smaller in diameter.

You can either open up the rims, or cut down the hub on your brakes/axles. If you run spacers on the rear that are wider than 1/4" you don't need to do it for the back.

Disc brakes in the front still have the issue even with 1 1/4" spacers.

I cut down my rotors in the front on a lathe to fit my rims.