The 3 speed overdrive transmission was a factory option for Chevy cars and trucks from 1955 through the early 70's.
The first version which would be a bolt in for your truck, is a Muncie SM319. (1955-65) It uses the same basic transmission as your truck with a Borg Warner R10 overdrive unit that replaces the rear tail housing. It uses a cable to lock it in or out of overdrive. The electrical system consists of a solenoid, governor, relay, kick down switch, and a wire loom.
Chevy came late to overdrive's, Studebaker, Ford and other mfg's had them in common usage much earlier.
Fred
