Just got a 73 Plymouth Valiant 4dr

Just got a 73 Plymouth 4 Dr 318 PS PB AC, loaded for a $100. But it needs a replacment rear end and I don't know which rear end it has. The 3rd member is blow out but the rest of the car is ok. I don't which rear end it has 8 1/4 or 7 1/4? Can anyone help with some Mopar enlightment on this subject. I can't look at it right now I'm in CA and the car is in AZ. I haven't had time to get out there I just told them I'll take it.