The standard axle ratio for a manual transmission '64 LeSabre was 3.42. A 2.73 ratio was one of three available on the '79 Deville. The difference in driveshaft speeds (assuming the same tire size) is 3.42 / 2.73 = 1.253. So that agrees exaxctly with your driving tests showing the speedometer is running 25% high.
The replacement driven gear in the THM400 will need 25% more teeth than the existing gear to slow down the speedometer cable enough. That is a big change, maybe outside the range of available driven gears.
The adapter gearbox might be the way to go. A lot better than changing the driving gear inside the transmission.
As a footnote, the factory in the early '60s sometimes supplied these external adapters to obtain correct speedometer calibration. The design standard is 1000 rev per mile on the odometer and 1000 rpm = 60 mph on the speedo.
What has been, can be again. (Bob Wills, 1942)
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