Note the old GMC "tractor" that looks very old to me, and should arguably be in a museum! Also the '41-46 with brush guard appears to have a second gearlever...for fwd?
I am sure that the wings are indeed original from the corrosion. It reminds me as I said of a DIVCO or similar Walk-thru ore what they called a Parcel Van. It looks as though the cab/body has been cut back and a replacement cab inserted.
It might be worth posting a note on the Old GMC Triucks forum:
http://oldgmctrucks.infopop.cc/eve
Oh! Canadian GMCs mimicked Canadain Chevrolets and Maple Leafs, and there was cross-use in certain models of Chev and GMC or Chev-based engines. The '40 GMC could be either Canadian or US-sourced.