The 4-speed auto used in mid-50's Chevy and GMC trucks was a Dual Range Hydramatic. It's the same basic trans the 54 and 55 model Olds, Pontiac, and Cadillac passenger cars had. They're scarce, but not impossible to find. The bits & pieces to re-fit your truck back to original condition such as the bellhousing, crossmember, shift linkage, driveline, etc. will be harder to find than a suitable transmission.
I recently found a few factory service manuals for the dual range on ebay, to help out another bolter who was trying to get one rebuilt. BTW, the Controlled Coupling Hydramatic, used from 56-64 or so in Olds, Pontiac and Caddy IS NOT the same transmission. From 1940 to about 53 or so, the Hydramatic didn't have the Drive Right range which kept 4th. gear from engaging until the engine was in danger of floating the valves. It was the same basic trans as the 54-55 models, but it had a different valve body, and very few interchangeable parts. The truck shifter gear positions were labeled 1-2, 1-3, and 1-4, indicating how many shifts were possible for a particular shifter position. Reverse also served as Park, when the engine was stopped. Good transmission, but a little jerky on the shifts. I rebuilt my first one at age 12, as a student in an after-hours class my father used to teach for other mechanics in Nashville.
Jerry
