From the year of truck looks like a 270 ,just my best guess .Like you said to really know you will have to read the numbers.
Military GMC 302 engine pictures
http://www.alfaheaven.com/MilitarySection/Engines/G21.html The GMC 270-cubic-inch (4.4 L) 270. It was produced from 1941 through 1962 and was an OHV/pushrod engine. This is a raised-deck engine.
The 302 GMC inline six was produced from 1952 to 1960, when it was replaced by the V6. It has a 4.00" bore and 4.00" stroke. Connecting rod length was 7.000". This is the largest raised-deck engine. It was originally designed for the GMC military M135 and M211. It was used in military 2.5 ton trucks with the HydraMatic transmission, however the engine was a sealed engine for snorkel/submersion use, had an electric fuel pump, and other features such as a deep sump oil pan. From 1952-1959, GMC manufactured the civilian 302 engine which was not sealed, had a mechanical fuel pump, and used a "standard" oil pan.
