1939-1955 for the 248 engine.
As posted above, a late 1955 engine might have been used in truck manufactured in 1955 but first registered/title on 1956.
Can you post the various codes on the head and block (there is a casting-number code and a date code on both the block and on the head)? The engine serial number should be pressed into the ledge in the block just to the rear of where the distributor enters the block (this should show the the engine size).
Here is the location of a date casting code on a GMC-6 block. I'll look for a photo with a
more complete set of code-locations (from the oldGMCtrucks site - Rob English annotated a photo with locations).
