what wheels are you getting? im not sure on the backspacing but ive got 315's, no fender rolling or spacers needed. oh yea, im on eibach pro kit springs... height could have something to do with whether or not you roll the fenders. what springs are you on, youre pretty low.
those should be designed with the proper backspacing for a mustang since they are a "cobra r" wheel. i wouldnt worry about getting spacers, you might have to roll the fenders though because youre so low :dunno: