Nick - I got a chance to do some digging in the GMC resource books I have on hand. Below are snapshots of what I think may be relevant info to try and help you answer at least some of your frame questions.
The 1939-40 GMC Shop Manual, 1939-41 GMC Book of Parts and 1939-45 GMC Parts Books show 1939-40 1/2 ton Model AC-100 trucks had 113-1/2" wheel bases. I think that rules out your frame being a 1939-40 GMC, which matches with your findings on the same Chevy 1/2 tons.
The GMC 1939-41 and 1939-45 Parts Books both show 1941 and later 1/2 tons Model CC-100 have 115" wheel bases. Again same as your Chevrolet frame wheelbase dimension findings.
I think the primary difference between the two frames (Chevy vs GMC) is the GMC front engine/radiator cores support cross member is farther forward than the Chevy to support the longer GMC engine. I think that accounts for your difference in the different distance from the cab and radiator since the GMC radiator is meant to be farther away from the cab than the Chevy. The different (farther forward) GMC front engine mount location coupled with the rear engine cross member will mean your Chevy 6 and transmission package are going to fall short of the front engine mount location, which I think you had been discussing in another thread.
As to the missing cross member between the rear engine mount and parking brake cable pulleys mount, IIRC that cross member is for the rear of the transmission and I think it's a bolt in (removable) unit. It may have been removed by a PO and not reinstalled.
Unfortunately none of my GMC books actually give frame dimensions like the Chevy diagrams you've shown, so I can't confirm if there may be differences in the overall length or width between the Chevy & GMC or not.
Perhaps there are some more knowledgeable GMC guru's that have the frame dimension details, or a current truck owner(s) that could take some measurements and post to try and help clear up what you have???
Sorry I couldn't be more help in clearing up the muddy waters.
