Hi Pete,
It sounds like you have a "Silver book" option available from your Chevy dealer.
My 1937 Ambulance came with the 9.00 X 20 option. They are called "Budd Duals".
The factory modified the running boards and rear fender to accommodate the larger tires.
There's a picture of the ambulance and wheel explanation at <
http://home.znet.com/t1937/ambl.htm>.
Email me if you want more photos.
Lou c1937@znet.com
The hole in the center of the wheel is larger after 1936 when they used a full floating axle.
The lug pattern is the same so newer wheels will fit older trux but all the weight is carried on the lugs and not the axle. .