Factory BBP on 7.25 axle
My '73 318 disc brake Dart Sport is BBP with a 7-1/4.
The '74 318 Valiant with disc brakes is BBP is an 8-1/4
The '75 318 Duster with disc brakes I had previous was BBP with 8-1/4.
I don't know what the criteria for axle choice was exactly.