Not sure if you'll be able to post any images yet with so few posts...you may be able to post a link to some-such as photo
bucket...would be interesting to see the door tag (could have saved a bunch of back and forth there) and the truck. Here's
a Dept. of the Navy truck I found on CList...has a code tag on the actual dash...second time I've seen these.

The seller may be suffering from a bit of heat exhaustion though with his current asking price for two wrecks.

1966 FORD "US NAVY TRUCK CARGO PICK UP

 

The military ran those trucks until they cost more to repair than they were worth sometimes they kept them even then depending on replacement costs. They could have changed the engine anytime. The only way to tell the difference between a 300 and a 240 is by measuring the stroke.

 

Added pics of ident plates in my garage.