You don't say what the back space is on the wheels. 15's should fit with no problems, as long as the back spaceing is correct. The back space is the width from the flat part of the wheel that goes next to the brake drum, to the back side of the tire side wall.
Lay a stright edge across the tire, and then measure the distance to the mounting surface. Now that you have this info, you should be able to look up on the web to find out what the max back spaceing you can use with out rub.
Try this http://www.mustangforums.com/