If it's got the 4-bolt typical flange, then the drumbacking plate should bolt on with the axle retainer. You should be OK as long as you dont have wierd axle lengths that would put the drum in/out too far.
My guess is either the disks are too large for the wheels he wants to use, or more than likely his currie rear is 5 bolt and he wants to convert to 4-bolt (again probably because of wheels).
Well, I don't think it has anything to do with 4 lugs, the likelyhood of Currie building a 4-lug disc brake setup has to be close to nil.
That being said, there's nothing wrong with drums in most applications. I know I'd much rather push a drum car in the staging lanes at the track than a disc car, and a disc/drum car with a disc/disc M/C really sucks.
I'd say probably not, most Mustangs used small bearing rears and most of the aftermarket rears are either big bearing or torino style. I would think it'd be alot easier to just find some backing plates from a big Ford with the proper bolt pattern and center hole...