I just sold my '87 Corvette and actually had this same idea - I was going to part it out and keep the IRS for the Mach 1. The aluminum batwing mounts are too wide - they are about an inch outside the framerails on my '70. You could fix it, the batwing can be narrowed, but I doubt if it would be cost-effective. I didn't measure how wide the four link was, but you might have problems with the location of that in relation to the stock Mustang frame. Probably have to shorten up the half shafts as well. The C4 is a pretty wide car and the wheels use a lot of backspacing (36mm offset on the '84-87 and 56mm offset on the 88-96). Plus, there is a C-beam that runs from the transmission tailhousing to the snout on the third member. Not a huge deal, but just another hurdle to overcome in mounting the rear in something other than a Corvette. Personally, I think C3 (68-82) rear would be easier to install in a non-Corvette application. The C4 Corvette rear is a very nice piece and there are guys running low 10 second 1/4 miles with them. The manual cars use Dana 44 ring gears and are probably the best way to go, so look for one out of a 4+3 (84-88) or 6-speed (89-96) car. The automatic cars use a Dana 36 ring gear and they're a decent piece too.
IMO, you'd be time and money ahead using the Cobra rear or even getting the Heidt's IRS, though that one's pretty spendy.
Hope this helps.
