According to this chart...
http://www.discountedwheelwarehouse.com/Ford___Vehicle_Bolt_Pattern_Reference.cfm
... Escort 15's started in 91, with a 4x100mm bolt circle. Your Mustang is 4x108mm.

The offset will be totally wrong too, as FWD wheels will have a lot more
"positive" offset (tucked in under the car) to keep them centered on the front
suspension "pivot" points to reduce torque steer, while your RWD Mustang
wheels can and will have "negative" offset (they stick out further) that give
you that "wide track" look.

You can try to cram them in there if you want, but there's a good chance they
won't clear your brake or suspension parts, and you'll have 4 goofy-looking
wheel wells with the tires & wheels hiding waaaaaaaay inside in the shadows.