The 7 1/4" axle was standard on every A body built. EVERY one.
The 8 3/4" axle was the upgrade for the towing package and could be optioned even on slant six models. The 340 cars from 68-72 got the 8 3/4". All base model slant six and 318 cars got the 7 1/4" axle if there was no towing, Police or Taxi fleet option chosen. .