With your car being a true cobra I'd take all the original brake parts off. You could get away with a fox length 8.8 you can use the 94 to 98 rear calipers, brackets, and anti moan brackets. A machine shop can machine the newer rear rotors ( 5 lug ) to fit the 4 lug pattern. The center bore also needs to be machined. Fox hub is 63.4mm and the newer sn95 is 70.3mm. The sn95 brackets just need to be swapped side for side. Left brackets and calipers installed on right side and right on left. The orientation changes from the rear or front of the wheel well....ahead or behind the axle. This would give you the same offset as a stock fox mustang.
The only thing that makes the super coupe .75 wider are the axles to clear the brake brackets. I believe your 8.8 and a fox 8.8 have the same ends. Meaning the brake swap for a fox should bolt the same way to that rear. Just need fox length 4 lug axles, rotors drilled to fit, and sn95 brake parts. All rear brakes are the same from v6 or v8 from 94 to 98 except the cobra ( they had vented rear rotors and wider caliper brackets but same caiper )
You can also use NorthRacecar brackets
https://www.northracecars.com/vintage_brakes.html
It's been a really long time since I've seen a turbo coupe rear brake set up.