What is there to be confused about? Decide WHICH brakes you want. If you want later A body discs (73-76) you need to go to the 4.5" bolt pattern. Unless you want to run 2 different bolt pattern wheels, you'll need to convert the rear to the large bolt pattern to match the later big pattern on the late A body disc brakes.
OR if you want to retain the 4" bolt pattern, use the 72 and prior small bolt pattern disc brakes on the front, and you'll have to do nothing to the rear.
Those are your choices. You can do either choice with factory used parts rebuilt, or aftermarket new parts.
What part about any of that is confusing?
