What is the model of the truck? I had an ex-CDF '40 GMC that had dual rear wheels (with extended fenders) and a 9 1/2' bed. It was actually a AC302 chassis with a 236" GMC, 4 speed, and 2 speed rear on 20" rims. It would carry all the road base you could load into it with ease, stopped as well as it ever did, and actually steering was easier with a couple of tons on the back ;-)
Tim