Steven,
Your dump truck is definitely a Chevy. It was a popular conversion to replace a used up babbit Chevy 235 with a GMC 270 or 302 along with the GMC Clark 5 speed transmission. Many (but not all)parts are interchangeable between Chevrolet and GMC WW2 wartime and Civy trucks and yours seems to have gotten some parts from a GMC donor somewhere in the last 70+ years.
The axles on your GMC COE are Timken "split" axles. They appear to be much the same as found on the early GMC 6x6 CCKW or "Jimmy" trucks.
The link below may provide some useful information.
http://vehiclesofvictory.com/chevy-g506/chevy-pictures/historic-chevy-photos/You have some nice old trucks, I hope you conserve them in the best way possible.
Paul
