The 86-98 model Mustang 8.8 rears are the same dimensions, only the 94-98 cars have 3/4" longer axles and corresponding disc brakes (for the optional ABS). The 87-88 Turbocoupe rear(4 lug) and the 86-92? Mark VII rear housings are also the same dimensions, but again with longer axles, 3/4" and 1.25" respectively. Any of these housnings will be a direct exchange in a Fox Mustang, the only considerations being the axle length and # of lugs.
The 99-04 Mustang housings are about 1.5" wider, making them about 3" wider overall with the longer axles.
If you got a 94-98 with rear disc and the longer axles, TC rear or Mark VII, it can work on your car, but you would need to be careful on your wheels due to the longer axles.