do you have the owners manual? not sure if it says so in the 'little' truck manuals, but my '59 6400 series gives both gvw and gcvw for the various models in a big table in the back. 19000 and 32000 for the 6400.
i'm not sure if trailer weight is the absolute of gcvw minus gvw, or if it can be heaving if the truck itself isn't loaded, but that should give a good minimum i'd think. sorry to not have a 'real' answer, that's as much clue as i've got
