Rear end swap /brake questions

The 79-93 Fox body rears are all 4 lug. You can convert them to 5 lug by using two long side Ranger axles. I would do an Explorer 8.8 though, the pinion offset is 4". You don't have to shorten the long side that amount, only enough to clear the driveshaft in the tunnel.

try an 80s model lincoln mark series rear end...they are not offset and have rear disc brake...my 84 lincoln 8.8 rear end is about 1 1/2 inches longer than the stock rear end, but thats not a problem for me...I plan on using late model wheels anyway

The Lincoln MK VII rears are about 1 1/2" wider than the Fox Mustang which is about 2" wider that the 65-66 Mustang. The only Fox Mustangs that came with rear discs were the SVOs ( 5 lug Lincoln MK VII parts on a Mustang housing) and the 93 Cobra (4 lug T-Bird Turbo Coupe parts on a Mustang housing).
The Explorer isn't a bad option at all. You only need to shorten the long side to match the short side and get another short axle, then your pinion is centered . Explorer rears are easy to find in wrecking yards and most of them have 3.73 gears and traction lok. You can also find 3.55s or 4.10s without much trouble. The best you can hope for in a ox or MK VII is 3.27s. All the 95-up Explorers came with disc brakes, too. It may not be the "best" option, but I think it's the best option if you're on a limited budget.