By far, the easiest way to convert a Fox body Mustang to a rear disc brake setup, while retaining your 4-lug wheels is to use the rear end out of an ’87 T-Bird Turbo Coupe.
This is essentially the same rear end used in the ’93 Cobra Mustang (the Cobra’s had different rear gears), so they bolt right into a Fox.
Depending on what gear ratio you’re looking for, you can pull a rear end out of a Turbo Coupe with an automatic or manual transmission. The autos had 3.73 rear gears and the sticks had 3.55 gears.
The brake line between the car and the axle is the only part that you can’t just swap over. But even the brake line is not that difficult. Purchase the rear line for an ’87 Turbo Coupe from your local parts store, and take your stock brake line to a local shop that does transmission work, brake work, or anyone else that can put together a stainless steel brake line. Have them put together the end that connects to the car on your stock line and the end that connects to the rear end from the line you purchased with about 6 inches of brake line. I recommend that you install the rear end, and use a piece of hose (or string, or what ever else you can find) to determine the correct length for your car and your suspension. The shop I used only charged me $20 to make the new brake line.
After that, it’s more or less just bolting the thing in.
You will find that the quad shocks do not align on this rear end. There are two options, drill a new hole, or dump them completely. If you install aftermarket control arms, or even just aftermarket bushings in your stock control arms, you’ll find that there is no need for the quad shocks. The stock suspension bushings on a Mustang were intentionally soft to make the ride soft enough for your grandmother. A result, wheel hop. The fix for that, quad shocks. Fix the root cause – the soft bushings – and you don’t need the quad shocks. Having used both, I can recommend Maximum Motorsports and Steeda control arms. However, if you absolutely have to have them, just drill a new hole in the mounting brackets 2″ from the current hole and bolt them up.
You really, really should make some updates to your brake system when you do this swap also. Modifications include swapping some parts on the brake proportioning valve (in the engine bay, under the master cylinder) and installing an adjustable valve. You can also easily upgrade your master cylinder to take advantage of the upgrades. While you can technically drive the car without doing this work, you’ll want to do these to take advantage of the rear discs and avoid any potentially dangerous issues (like not stopping when you want…). I’ll try to cover these modifications in my next article.