yeah i have the same car, and you need a pretty narrow rear end...52". I have a 9 inch that came from a 65 mustang for mine and Im scrapping the 8 incher. Pretty much anything you find with disk brakes youre going to have to shorten or do some kind of mods. Let me just tell you, people always say the lincoln versailles rear end (9 inch w/disks)is a direct swap for mustangs, but its not, it too needs modifications for u joints and spring perches. Well good luck finding a 9 inch thats a direct swap, im the luckiets man on earth to find an origional one, it had to either come from a k code 289 or a shelby. 8 inchers cant be hard to find, all mustangs came with one (except 6ers and hipo mustangs) but they'll come with drums cuz they didnt put rear disks on mustangs till just recently.
If you can find an 8 inch from a falcon that would be a good bet also, they're a bit narrower which allows the use of bigger rear tires. but again, out of luck for disks. If you have money to spend i would recommend wilwood disk brakes, i have them on the front of my stang and they work great soon ill get some for the back