[rotorwrench]

Four speed gearboxes were only used in trucks prior to the late 1950s when the T10 came out. They are huge, plus most are non synchronized. The only option would be the Mercury R10 overdrive transmissions of 1949 through early 1951 with a top shift adapter installed. One of our members (expavr) did this with success. There are posts about it on this forum. The other option is the Columbia overdrive or a modern torque tube overdrive like the Mitchell overdrive unit. The older R10 and Laycock types are still around to some extent and they are electric where as the Mitchell is lever operated.