I know you can put a car up on the jacks and spin the pinion 1 revolution and measure distance the wheel rolls to determine gear ration. I'm a shade tree mechanic you know. Anyway, how can you tell the difference between an 8 3/4" rearend and a 9" rearend? I think I got two lockerboxes but not sure. Also I've heard of limited slip and Posi rear ends, but is there such a thing as a semi posi in the '71 to '73 cars?