The stock Explorer width is okay for a swap into a 67-70 Mustang, but as stated above, the brackets need to be relocated or removed and the pinion is offcenter. I've read that some have used them without any problem and others have had occaisional rubbing problems on lowered cars or on hard bumps. I think if you swap the Explorer pinion flange for a smaller diameter one off a Mustang it would solve the rubbing issue.
A lot of people will tell you to keep looking for a 9" and that there are still deals to be found. I'm one of them. However, it also depends on how patient you are and how long you're willing to look. In my opinion, though, you won't likely find or be able to build a similar 9" rear for the same price as an Explorer rear. Explorer rears are everywhere in junkyards for cheap. The majority are 3.73s w/ traction lok ("posi" to some), all are 31 spline, and all 95-up models have disc brakes. The disc brake models can be bought for $250 or less, and the earlier drum brake versions can be obtained for under $100. Grind off the old brackets yourself, pay a shop $100 to weld on new spring pads in the proper location and you're done. I seriously doubt anyone can duplicate this with a 9" for the same amount of money.