1991 Mustang lx/gt guage cluster ?

Yes, big differences.

Unless you want to swap the steering column to match, do without windshield wipers, and re-pin or solder wiring harnesses, stick with the style that belongs....

'87-'89 or '90-'93

If you're asking because you have an '85 MPH speedometer in you '87-early '89 and you want 140, 160, or 200 (Saleen), get one from a late '89 (starting October of '88). There are plenty to be found. (I just found one cluster and one speedometer on ebay.) If for some reason you can't find one, you can use a '90-'93 speedometer but you will either have to monkey around with mounting it inside the early cluster or remove the "guts" from the late speedometer and put it inside the early speedometer housing, and then get a 140 face to fit.