I thought I'd share some thoughts on my '55 second 3600. It had dual rears when I bought it from a body shop- they had been using it as a tow truck since 1957. It had 6 16X6 split rims- which were just normal offset rims. I've taken a picture of the rear hubs to show how the rears were mounted. The hubs are very heavy, and the bolts have flats allowing them to be tightened by wrench to the truck before adding on the outer wheels. When I changed to modern duals I needed adapters(of different depths) for the front and rear to position the wheels correctly. This seems in line with OlBlue's reference to the parts book but orchidskip's rear duals are hub centered in 1957. It's entirely possible that my truck had 17" single wheels which were also rated at 6900 lbs. GVWR and then converted sometime after it became a tow truck in 1957. There was a handwritten bill of sale under the seat that described the truck as a "1955 Stake Bed". If the hubs were indeed the same- single or dual in '55, I wonder if the mounting configuration that I found on the rear were actual Chevy parts.
Kim
