Here's what we did. Drove the car with a GPS showing the actual speed. At actual 40 mph, speedometer reads 60. At 50 mph speedo reads 75, and at actual speed of 60 mph speedo reads 90. In each example, divide the actual mph by the reading of the speedometer. In this case, it works out to a .67 reduction to match the speedometer to the GPS. Having a ratio reducer built since there is no factory gear available to correct the difference.
This is an early (63 BB 727) transmission. They are different from the '66 up units. Has an 8 tooth drive gear and a 16 tooth gear on cable now, car updated with 3:73 rear gear and tire at 26.5" diameter. This should work. It may be a little pricey, but they are the only game in town for the 62-65 trans.