Your going to have to use, at least, a 23 tooth (white) with your combination. This is the largest so it will still be off a little, but it is the closest.
The 1983-89 with T5 transmission used a 7 tooth drive gear vs the 1990-95 used a 8 tooth drive gear. That's the reason a 21 tooth with 3:73 will work for the 1983-89.
Here is a chart so you can see:
http://www.gatineaumustang.com/FTspeedogears.htm
Brad
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1991 Mustang GT Conv.
K&N air filter, Cobra manifold, 65mm TB,
Adjustable fuel pressure regulator, 180 thermo., Mac headers,
High flow H-pipe, Force II W/2 chamber Flowmaster,
Hurst Shifter, 3.55 gears, subframes
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