Here's the link to the
1948 Chevrolet Truck Data PDF at the GM Heritage site.
The screenshot below is an extract from that document, showing your AFCM is a Regular 216 built at the Tonawada Plant and originally installed in 1948 3/4 (3600) and 1 ton (3800) trucks. The 313991 is the engine serial number which started at 1001, making your engine the 312991st 216 built at the Tonawanda Plant for the 1948 model year.
There should be a make/model data plate affixed to the drivers door opening front side that will give more details on which model truck you have.
