91-93 Fender on 89 Mustang

The fenders will fit the problem is the front part of the fender where the fender extension goes is 3/4" shorter in lenght on the 91-93 than it is on the 87-90.
You'll also need different splash shields for the 91-93.

If you reuse your fender extensions on the newer fenders, they will fit, but you will not have any more wheel clearance. The newer extensions are cut back to fit the newer fenders and that is what gives the extra room for larger tires.

The fenders, fender extensions and slash shields are different on the 87-90 versus the 91-93.

The fenders and fender extensions on the 91-93 are 3/4” shorter in the front of the fender than they are on the 87-90.
I know this because I measured the difference between my 89 and a friends 92 GT, so unless you try to some how cut down the 87-90 GT fender to make it fit the 91-93 fender extension there is no way a 87-90 fender extension will fit on a 91-93 fender because it is 3/4" to long for the 91-93.