I'm just going from memory here, so take it with a grain of salt: The auto trans equipped trucks had a badge on the fenders that said they were automatic. I believe the tranny GM used for the Chevy trucks is the same Hydra-matic that was used in Olds cars of the 50's. If there's a tranny rebuilder where you live with gray hair he could tell you exactly what to look for.
These were great trannys. No overdrive, but a "digger" first and second gear. My grandpa's '51 Olds could burn rubber clear across the high school parking lot but don't ask how I know that.
