Congrats... Awesome find!
Based on the condition of the cylinder head under the valve cover, it does not look like it has seen much use.
Would bet that engine is from/for a DUKW (aka "Duck") 2-1/2 ton amphibious truck.
Many features are consistent with that application:
Four blade fan with reverse pitch (DUKW pushes air through the radiator as opposed to pulling it).
Shallow oil pan. Engines with later features such as on this engine (6 valve fuel pump. cast iron carb., distributor shield) would have the deeper two piece oil pan if used in a conventional truck.
Also not sure, but I believe that the adapter bracket on the front damper is for the compressor in the tire inflation system.
Best Regards...
