MickyP, you might not think your truck has been lowered but it looks a little low to me. Maybe it's the angle or that it isn't sitting on smooth concrete. The distance between the ground and the bottom of the door sill/running board should be about 13".
Looking at period pictures of '56 3100s, I think the front fenders and running boards should be just at or slightly above the center of the hubcaps.
According to the factory specs, the '56 3100 "platform height" is 27.82" unloaded and 25.95" loaded. No definition for platform height is given but I think it's from the ground to the top of the frame.
