Cliff notes version: (assuming you have stock 70-72 disc setup now)
1) 77-81 B/R body Rotor (Cordoba, Magnum, St. Regis) & rear wheel bearing
2) Dr. Diff bearing spacer
http://www.doctordiff.com/3) 77-78 Caliper adapter (pin type)
Just hang up your calipers, remove your rotor and caliper adapter. Install new caliper adapter. Slide the bearing spacer on. Slide new rotor on. Re-install your caliper.
One caveat, the rotors will require a 15" wheel!
I run this setup on my 71. Plenty of brake for the car.