4 wheel power disc brakes in 73 duster

+1 on that

You can use brakes from a 94-98 grand Cherokee. They are 11" rotors.

Hit a junkyard for the calipers and either reuse them (with new seals) if they are good, or use them as a core (Core is $40 each at O'Reillys). Also get the backing plates from the junkyard and be sure to take the parking brake stuff with you too.