They are both 8.8's but I'm sure the explorer rear is wider and has different mounts. The gear sets will interchange though, and I think a lot of Explorers came with 3.73s.
Once you figure out how to mount leaf springs in your mustang, remove the quadra bind 4 link system, then move the drivetrain over 1" in the car, the only thing you have to worry about is the different lengths of the axle since the left and right side are not the same length like your stock axle is.
The Exploder rearend is about 58.75" wide wheel mounting surface to wheel mounting surface. As was already said the center section is offset so the axle shafts themselves are different links. All solid axle explorers come with 31 spline shafts. 96 and later explorers are disc brake rear. Popular gear ratios where 3.55 and 3.73 but 4.10s could be had with the towing package.