A lot of people don't realize that the 8.8 housing is basically the same from (86?)87-98. The difference is the disc brakes and longer five-lug axleshafts on the 94-98's. Point is, you can get a Mustang housing from any V8 car from those years and it will bolt right in, because the housings are the same width. If you don't want to convert to five lug wheels and rear discs, the axleshafts and drum brake assemblies from your 7.5 will all fit the 8.8 housing with no mods. Take note that the housings for the 99-04 cars are wider and should be avoided if you're looking for easy bolt-in.
An added bonus is that the V8 Mustang rearends all come stock with a limited slip--something that your 7.5 probably didn't have. If the limited slip is worn out you can buy a rebuild kit from Ford Racing for about fifty bucks.
I'd try and find a rear from a late 80's turbo coupe. You'll get disk brakes and either 3.55(manual donor car) or 3.73(auto donor car) gears. There was a great site a while back about the install, but i think it went down a couple months ago. Install didn't seem to hard, tho you have to change a few things for the disk brakes-but i'm sure its worth having disks in the rear.
Auto trans 87-88 Tbird Turbo Coupes come with 3.73 gears and manual Turbo Coupes come with 3.55 gears. Cost is $125-$300 for the rear axle. Add another $100-$200 or so to complete the brake upgrade.
I choose 3.55 since I do more highway driving. Both ratios have 10” disk brakes with vented rotors as standard equipment.
It takes 2 guys the first day to get the old rear end out and the new one bolted in place. It takes 1 guy another whole day to do the brakes.
You will need a several sets of fittings, I recommend that you get them from Matt90GT's website, http://www.svo73mm.cjb.net/. Read Matt's instructions thoroughly, everything you need to know about the brakes is all there. You need to be patient and follow all the internal links, and there are many of them. You will need 2 fittings in the rear to adapt your old brake tubing to the TC disk brakes. The fittings go between the steel tube and the caliper brake hose. You will need another set of fittings to make a 2 port to 3 port adapter. To make life simpler, just buy the kits from Matt. You could piece them together, but it's not worth the time unless you work at an auto parts store with all the fittings ever made.
You will need to drill the quad shock mounting holes 2” below the holes drilled for the Turbo Coupe mounting points. The bolts are metric, so don’t loose them or the nuts. A 15/32” drill should be about the right size unless you have access to metric sized drill bits. Going without quad shocks is not an option unless you have aftermarket parts to soak up the wheel hop.
You will need a proportioning valve, Summit has one for $40 + shipping.
You will need a kit (FMS makes the part) to gut the stock proportioning valve, Summit also has that, about $10.
You will need a new master cylinder, see Matt's site and make you choice. I used a 94-95 Mustang master cylinder. Note that rebuilt 94-95 Mustang master cylinders do not come with a reservoir. That means a trip to the junkyard and some more money spent.
Your brake pedal may be very hard and almost impossible to lock up the brakes. I had to replace the front calipers with 73 mm calipers from a 91 Lincoln Mark 7 to get the braking performance up to par.
Bleeding the brakes will require 2 people and some coordinated effort. I don’t recommend using you wife or girlfriend to pump the pedal – they get offended when you yell at them. I used a homemade power brake bleeder constructed from a garden sprayer and some fittings from Home Depot. It cost about $25 and was worth every penny.