Hey reach out to crutchfield.... they always are very supportive online and via telephone.
Here is what they mention for double din install to 91 lx coupe...
Installation Details for your 1991 Ford Mustang Coupe with 6 speakersTo install this item, you'll have to modify your vehicle.To create enough space for a double-DIN (4" tall) receiver, you must remove the factory amp's bracket and cut off the metal studs on transmission tunnel.You'll need to drill four holes in your console, but your mounting kit will cover these holes. If you re-install the factory radio, you can cover the holes with plastic plugs or painted screws.