Something else I just remembered is that the '83 M/C has 2 ports with 2 lines running to the proportioning valve. The '88 M/C has 3 ports with 2 lines running to the proportioning valve and the 3rd line running directly to one of the front brakes. If you're pulling all these parts from a parts car, it would probably be wise to go ahead and swap the entire brake system if you have it there. You're not really gaining anything performance-wise by swapping only the M/C anyway. The '88 (assuming its a 5.0L car) will have the larger 11" front brakes, you're '83 has 10" front brakes.
1985 Mustang GT 5-Speed - See Profile for Mods & Pics