Your 90 would not have been 3.55 from the factory, all 87-93 Mustang 5.0 had 2.73 or 3.08 as a no-cost option. It did not change with the 16" in 1991 since the overall tire diameter was the same. If you get the axle code from the door VIN and info decal you can verify what it was originally.
cheers
Ed