Here's my guess if the differential wasn't changed out over the years.

If it has 15x4 brakes (15K axle) it could be a 6:40/8.72 or a 5.83/7.95.

With 15x6 brakes (17K axle) it could be a 6.40/8.72.

These were the popular two speed axles during the mid sixties on GM medium duty trucks.

SimS