s.b.p. disc to b.b.p. disc front conversion 72 demon

It sounds like you are thinking of buying 1973+ Mopar BBP parts (post #3). If so, you will need matching spindles. Not trying to sabotage barty's offer, but you should know the full story.

Using Mustang hub/rotors to get BBP (post #2) sounds easier and cheaper, since you keep your spindles.