Today's fruitless search for Bullitt wheels, LOL

You should be able to fit 17x8's or 18x8's with no problems. The backspace on the mustang rims would require a spacer, but it should be fine.

Without moving or offsetting the springs a 255 would be about as wide as you could go, and with a 17x8" you'd need about 4.5" to 4.75" of backspace, which usually takes a 1" spacer on most the mustang rims.