The 2nd Edition of Motor's Repair Manual illustrates a water pump on an F-33 with a 286 engine. The 286 appears to be a flathead with a 3 5/8 bore by 4 5/8 stroke with a 4.90 compression ratio. 90HP @ 2600 RPM. It looks like 1938 was the last year it was used which is about the time that type of engine became obsolete. However, it is very possible that it may have been repowered with a later model engine. Truckers have always done that. The picture of the engine you post is an overhead valve. I have strong suspicions that the truck does not have the original motor. It might not even be a GMC motor. It could be something like a Waukesha.
