Really interested to see what the outcome is. Great looking car. I had a 77 that was the same colour. Since you have a posi, it's a bit easier to check. Put car in neutral, parking brake off. Jack both wheels off the ground (support with jack stands). Crawl under car, mark driveshaft and inside of one wheel. Turn the driveshaft by hand while watching the mark on the wheel. Count how many revolutions of the driveshaft it takes to get one full rotation of the wheel. If it takes just under 4 driveshaft turns to get one rotation of the wheel, you are probably still 3.73. If you are over 4 driveshaft turns, somebody charged the ring and pinion at some point. Make sure both wheels turn when you turn the driveshaft. If not, you will have to hold one wheel with your foot to keep it still, and double the amount of driveshaft turns to get one rotation of the wheel.