any input

Most likely a 8 1/4, but its also not an A body, so you would have to have it cut or something. There are plenty of A body rears out there that would be as close to bolt in as you can get. I would look for one of those.
340 are awsome, but expensive to build. I would go with the 318 or 360. Also remember, most 318, 340 and 360 part are inter changable. 360 heads on a 318, same rod size etc.