From an old article about Advance-Design trucks:

. . . In 1948 Chevrolet was the first of the volume manufactures to offer a 4-speed synchro-mesh, helical gear transmission as regular equipment. It was an extra cost option. As a result of this complete redesign, the 4-speed transmission case was wider, higher, and longer. It was still of the same rigid, box-like cast iron construction as before. So that the new 4-speed transmission could be mounted interchangeably with the 3-speed transmission it was necessary to increase the length of the 3-speed transmission case and main shaft. . . .

Here's a diagram of the rear-axle, for part numbers.
It looks like the torque-tube changed in '48.

Tim