First requirement for the Explorer rear disc swap is to have a large axle bearing rear end housing (Most early Mustangs do not have large bearing housings --they are medium bearing housings).
Second requirement is 3/8" diameter housing end flange bolts (where the stock drum brake backing plates bolt to the housing ends). Not ½" diameter like is found on most large axle bearing housings.
The mounting pattern must be 2" vertically x 3.56" horizontally for the Explorer swap. Most vintage Mustang's end flange bolt pattern is 2" vertically x 2-3/8" horizontally.
Third requirement is the axle must have a minimum distance of 2.088" from the wheel flange of the axle to the start of the bearing journal surface on the axle. Most vintage Mustangs fall into the 2.090" range.
There is an alternative to the Explorer and Versailles rear disc swap though:
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=11