I just exchanged some emails with Karl Schaeffer. He said he is often credited with more info than he has.

He said.
"I had some discussions with a online group that restores old GMC trucks and all I have is their opinion that these were used in heavier trucks and not in the 2 1/2 ton trucks. There were only a few thousand made between 1939 and 1947? Ours had 1947 cast dates and appears to have been painted olive drab. It is reasonable to assume that the RGS got them on some kind of surplus deal."

He also said The only primary source he has is the GMC manual for the 361.

possible scenario to explain this.
The engines were built for general use in the civilian word, but GMC might have been trying to persuade the military to buy them. And use them as a replacement/field mod for the older smaller straight sixes.

Perhaps the military bought a few as a test, but they didn't preform very well or were not enough of an improvement. So they sold them. Another right place, right time thing.