With my work trucks I can license them anywhere between what the net weight of the vehicle is and the maximum gvw that is listed on the door tag. According to the DOT it is illegal to license the truck for anything over what the truck is built to be. I've asked the question about adding gvw capabilities to a truck and was told that if the gvw is changed an appropriate addendum tag needs to be added to the door of the truck. I was looking at my 2006 Sterling yesterday and the company that installed the van body on it even added a tag to the door jam, altho the gvw didn't change.
I would think that you could find the weight in a salemans guide or another dealer publication, then contact a body builder with the type of body/bed you have on your rig and get a number thats close enough to get registered.

Good Luck
Don