How are the brakes and suspension? Do you plan on some chassis work like subframe connectors and a shock tower brace?
You got me @FoxChasis, guess I need to type faster
After that you can plan on gear change, maybe a heads, cam, intake and carb update, it is a carbed car not cfi right?
The MM brake upgrade is for 1987-93 Mustang 5.0L, and only includes pads, shoes, lines, and sleevs, so everything else (spindles, rotors, calipers, bearings, and seals) from a 1987-93 Mustang 5.0L is needed.
Don't forget proper wiring for a high-output alternator. The factory wiring back to the battery isn't going to cut it and the plug/harness and external regulator need an adapter.
I personally would not bother putting gears in a 7.5" rear. I'd rather swap to an 8.8" rear and then do gears. The 1985 7.5" rear is a bit stronger that the 1984 and earlier units but no where near the strength of an 8.8"
Personally I wouldn't spend a dime on brake parts until you've decided if you want to stay 4 or go 5 lug. Either way I would suggest a full disc setup with upgraded rotors and calipers up front- those 85 brakes are hot garbage.
Oh and I also support a swap to an 8.8" rear before you do gears.