You can fit just about any rearend on your stang if you want, it just depends on how much you want to pay to have it modified. I would recommend getting one that will bolt in with little (if any mods). In general, the most difficult is finding one where the leaf spring perches line up. You should measure the distance between your perches and total width from end to end and compare it with the rear you are looking at. Keep in mind that you may need to change the yoke or modify the length of your driveshaft. The brake lines and ebrake cable locations should also be considered.
Any rearend from a 67-70 stang will bolt in with no mods. 65-66 are too narrow and 71-73 the perches dont line up (i think). Some cougar rears will bolt in too. You can also look for a rear from a 76-79 granada/monarch/versailles, but thats another story.