GM 12 bolt truck axles from the 60s will also work... the backing plates are the same as the eaton HO33 axle. The barke cables will interchange. You will have to have new spring perches welded to the axle.
The HydraMatic is a 4 speed automatic. The linkages are finicky on them. Is your transmission shifting through all the gears? If not, lower speed could be expected.
How are you calculating speed? Stock speedometer? After 50 years a number of factors can combine to make the factory speedometer inaccurate. Taller or shorter tires can add to the inaccuracy. In the 21st century, gps units can be used to get a reasonable idea of how fast you are traveling. There are other meathods that can be used as well such as the measured mile and the stopwatch.
