olddozer,

Maybe it's not a GM engine; maybe it's a mistake by the farmer? And, you are correct about many farmers being able to do all sorts of things to keep these old trucks running. That says something good about farmers and about these old trucks.

The flatness of the head does not make a engine a "flat head". See if the spark plugs are on the top of the head (most likely a transplanted flat head, with "valves in head"); or, see if the spark plugs are on the side of the head ("overhead valves").

Tim