Agree with these guys, with a 360 and auto most likely an 8 1/4 (if everything is original) but 8 3/4 were optional and very possibly could be in there. Look at the rear, does it have the removable center pumkin, or does it have a cover on the rear (8-1/4.....10-bolts, evenly spaced, cover is almost perfectly round)? I highly doubt that any B body ever came with a 7 1/4 rear, not saying it's impossible, just very unlikely. I have had a 73 and a 74, the 73 had a 400 with auto and had an 8 1/4, 74 had an 8 3/4 behind a stout 318 and 4 speed (wasnt original that I know of though).
