Note that the STDB that John links to is a reprint of the original 1949 STDB.
Originals for each year sometimes show up on eBay (and the prices rise every year).
JC's note is a little misleading: "1949 Salesmen's Pickup Truck Data Book Same for 1947-1953 Chevrolet and GMC Pickup Truck"
It is not just a book about Pickup Trucks (it has information on all truck models)..
It does not cover differences over the 47-53 years, but there were very many similarities over those years.
It is not a GMC STDB, but there are many similarities, except for frame, wheels/tires, front body parts, and drive train).
"Question for the experts: If the SM-420 was optioned originally, then it would have an enclosed "torque tube" rather than an open driveshaft, yes?"
I am not an expert, but, "yes" if the SM420 was installed in a torque-tube (closed/enclosed driveshaft) truck model, the torque-tube was used with the SM420 and the "3-speed" rear end/etc.