I agree - the frame would have to be lengthened to accommodate the tandems. I have never seen one - looks like a pretty clean job of it. I'm guessing this was a pumper-tanker, and the tandems were necessary to carry anything over 500 gallons.
It begs many questions: would you use a 2-speed rear end? (almost definitely, in my mind). Brakes on both axles? (you'd sure want them!). Any connection to the military tandems that were offered in WWII and after by GM? Those were true tandems, tho, not just a tag.
Looks like maybe the truck was made by General, in Detroit? Just a guess . . .
-Michael
