That "extended cab" was a early built in sleeper. Actually not much more than a shelve to lay on with some storage underneath. Not many trucks had a sleeper back then as they were mostly used in somewhat local trips that could be completed in a days time. Not many coast to coast runs back then, if there were , they were shuttled between trucks at terminals spaced out every so many miles. Most of the trucks were narrow cabs, so you had to sleep curled up, no stretching out. Lots of guys who did travel overnight slept in or on the trailers, or carried an hammock and swung it under the trailer, no AC cooled trucks back then. Actual usable sleepers didn't start getting common until the mid to late 50's. Even then they weren't much more than 36" wide, if you had a 42" wide you had a big one.
My current truck has a 72" sleeper tall enough Shaq could stand up in comfortably, as well as lay down and stretch out.