I don't think there is any way to legally say a what a truck is 'rated' after it has been modified from factory, especially if front springs are used in the rear and not a factory single axle rear suspension (like an air-ride suspension)
I can say on the 939 series dataplates that the front axle is rated for 10k lbs. I can't imagine anyone buying a bobbed truck to use it for heavy hauling, but the 5-ton springs should easily handle 5klbs in the bed of the truck I would imagine, but putting any 'legal' number on it would probably not be possible.
DO NOT buy a bobbed 5-ton that someone built using M105 trailer springs, I would stay away from any using deuce springs as well-just my opinion.