'87/'88 Turbocoupes have an 8.8" rear with disc brakes and either 3.55 (w/ manual) or 3.73 (w/ auto) gears. The housing itself is the same width from axle tube-to-axle tube but the axles are each ¾" longer, for a total of 1½" axle flange-to-axle flange. That may or may not cause interference with the quarter panel's wheel well lips, depending on the backspacing and offset of the wheels you use. If there is interference, you'll have to use different wheels OR roll the lip or cut it off. Either of those last two options (roll or cut) is a hack job, in my opinion.
You will also have to monkey around with the steel brake line on the chassis and the flexible hose going from it to the axle housing. The pre-'85 V8 Mustang's chassis line ends at the passenger side frame rail, then the flexible hose runs from it to the passenger-side axle tube, which connects to a junction bloc/vent that splits into a hard line for each side. After '85, for V8s, the steel line crosses over to the drive side driveshaft tunnel, then the flexible line goes to the top of the differential housing on the driver side, and splits to each side from there. The vent is in the sa,e location as earlier years. The '87/'88 Turbocoupe is the same as '85+ V8 Mustangs. The Turbocoupe has a short flexible line going to each caliper.
OR you can swap the axles, drum brake system, and hard steel line from your '84's axle housing to the Turbocoupe housing. You will have to remove the Turbocoupes's axle tube vent and replace it with the vent/junction from the '84.
If your '84 has the Quadra-Shock rear suspension and you plan on keeping the shocks in place, you will have to drill a hole in the Turbocoupe's axle-mounted brackets 2" lower than they are now.
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