Big Bolt Rears

So I'm at the junkyard today (Englers, NE PA) and i mention to Bob that I'll be needing a BBP rear to go with the BBP front I'm converting to with my disc brakes. He mentioned he had a number of BBP rear ends on early 70's Darts and Dusters that had 318's.
Question...what size rear end did Chrysler mate to 318's? I'm thinking it's probably a 7 1/4 or 8 1/4?