That too-quick shifting is a throttle pressure issue, probably a piece of linkage from the carburetor to the transmission disconnected or out of adjustment. Look for a dual-range Hydramatic service manual online, preferrably one for the correct-year truck. They were used in a lot of applications, both cars and trucks, so any Hydramatic manual from about 1953 to 55 on the passenger cars, or from 1952-61 or so on the trucks should work pretty well. They were used in Olds, Pontiac, Cadillac, Nash, Chevy and GMC trucks, and some late 40's and early 50's Lincoln applications. Good transmissions, but tricky on adjustments!
Jerry
