I have never seen factory BBP on an a body unless it was a disc brake car. Discs were BBP and drums were SPB on every car I ran across.
I've seen a few A-bodies with B/E body brakes on them so that's probably what they came off originally.I had a 68 Dart GTS that had LBP drums on the front when I purchased it.
I still have the drums with the hubs and spindles for sale.
Ed
The B-body hubs should work on post 73 A body spindles. That would solve that BTW I still need BBP hubs and late A spindles.