Old to newer

73 - 76 A-Body disks are a direct bolt on. You need the upper control arms, spindles rotors etc from the donor car. Master cylinder, booster (if desired) and associated plumbing swaps as well. This will convert the front to the larger bolt pattern found on the B, E and later A-Body cars. Swapping the rears can be as easy as swapping out axle shafts and brakes (7 1/4" axle) or if your donar car has an 8 1/4" swap the whole axle assembly. Heres an article that esplains it pretty well: http://www.moparaction.com/Tech/archive.html

Some people have used the F, M J-Body stuff such as Diplomat parts, but the spindles are taller causing potential grief with upper balljoint bind. The rear ends in these cars is enough wider that swapping into an early A-Body might not be practical. There are some aftermarket setups out there which adapt GM calipers as well, such as www.scarebird.com