Can it work? Technically yes. Will it need major modifications costing more than the price of one out of a Mustang, most likely. I believe they had leaf springs, not coils. You'll need to weld on some shock mounts, who knows what else. A 91 is nearly 20 years old and it'll come with the problems of a 20 year old car part. I'd try to find one from a 94-04 Mustang. It'll bolt right in, and be in better shape. They don't cost that much. I paid ~$300 for one with aftermarket gears and did the swap in my driveway.
2011 3.7L V6, AM/Bama tune, 3.55, Flowmaster 40s
2000 3.8L V6, firstname.lastname@example.org all motor [SOLD]