Disc brakes were an option on all '73-6 A-bodies. All V8 models came with them. Any that came with disc brakes would also have a rear axle with the 4 1/2" bolt pattern wheels. They could have either a 7 1/4", or, 8 1/4" rear axle however, so, you'd just have to look under the car to see which one it has. Either will work, but, the 7 1/4" one is inferior in strength.