I don't think we should put this thread to bed.
I think its a great mystery and hope that someone can solve it.
Knowing what I know, I suspect that if the 361 came from a vehicle, Im betting it was a standard civilian cargo truck. The military bought alot of them over the years for state side use. Some were painted olive drab, others weren't. I have seen a manual for a GM panel truck, like a modern big delivery truck. Don't remember the motor, saw the manual several years ago.
I think we should all keep it in the back of our mind and someday we may stumble into the rigth guy or book that has the answer.
I for one will continue to pick the brains of people I know in the military vehicle hobby.