The Canadian 1948-50 Maintenance Manual Supplement shows both the Canadian Chevrolet and GMC offerings used the same Chevy 216 engine, so it can be presumed that both trucks used the same frame and drivetrain. Your assumptions about why GM of Canada did what they did could be valid, but we'll probably never know for sure.
Your "C370137" engine serial number matches known Canadian serial number formats. Somewhere I read that the "C" designation was given to the "Light Commercial Truck" 216 models, but I've lost the link to where that came from unfortunately.
I too have unfortunately not been able to find information on decoding either the trim or paint codes, yet.
