Wheel Question

As long as the tires you are running are the same overall size, it won't hurt anything, and some exotic cars come with this setup, though I can't really figure why you would want to do it on a Mustang..

Tech note: If your tires are not the same circumfrance it can cause serious braking problems, esp. if you have ABS. ie: 275/40/17 and 245/45/17's can be used together with no problems because the circumferance is the same (+- 1%). Not a recommendation, but, some of the online tire shops have calculators that will help you get the sizes matched.