Au contraire. The F/M/J spindles are a perfectly viable option to convert A bodies with. You still need the big ball joint upper control arms from a late A body in order to use them.
On an FMJ car you can pull the spindles and everything out from there and use that as a BBP disk conversion. In fact I’ve done exactly that on my Duster, although I’ve upgraded brakes again since then.
The later B/R body brakes can also be used with the 73+ spindles, and you can get 11.75” disks from those.
All of that’s covered in the articles @Professor Fate posted.
You can also use C body and (some) truck 8 3/4 axles and have them cut down and re-splined for A-body 8 3/4 use. Although DoctorDiff axles work better for that and aren’t so much more expensive when you consider shipping axles to Moser or something like that.
Rear drums/backing plates etc can come off of any BBP RWD car, some of that stuff carried over until ‘89 IIRC.
