Since this is said to be an original truck with an untouched engine, the combination of 1CM heads and a three-bolt carburetor has always bothered me. How about this scenario? The trucks had lower compression heads (8RT) than the cars (8BA). Maybe they ran out of 8RT heads and substituted 1CM units. The concurrent year Mercury's had a larger chamber because of the longer stroke. Everything turns out as it should, and the line keeps running.
Unfortunately, that would make it a 239.
I think you going to have to measure it to satisfy all of the inquiring minds out there.