Sampson

Need some help figuring this thing out. 1972 Cutlass 10 bolt (cover) rear end. Not sure if it is original as the engine and trans are out of a 1970. Car is and was a 350 automatic. Is there anyway to tell from the case if this is a 7.5 or a 8.2 or 8.5 etc. Also what am I doing wrong? I am turning the drive shaft yoke and counting the revolutions of the yoke to tire. There is no slippage but the driveshaft only rotates approx 1.25 times to each revolution of the tire. I know there are no 1.25/1 rear Axle ratios. Plus this car accelerated pretty strong. I felt like it would be a 3.08/1 just by the feel. Any help would be appreciated. I am trying to find out what it would take to upgrade this unit to a posi 3:48. Or do I search for a new rear? I estimate the engine will produce between 325 and 350 hp when complete. I don, t want to blow out the rear.