Hi Ulf...
I have a somewhat similar truck, a '57 6400 series with a 12 ft. steel floor flatbed dump. My truck weighs 8500 lbs empty, the GVW is 19500 lbs giving me a payload capacity of 11,000 lbs. Your 6500 series should be GVW 19000 lbs or more. Your truck, being a bit longer than mine plus a heavier bed should weigh around 9000-9500 lbs. empty, giving you an approx payload of 5 tons, (10,000 lbs.)
To be completely safe make sure your tire ratings in total will meet or exceed the GVW you have. Get a true empty weight on a set of truck scales when you can. And the obvious....have your brakes checked out thoroughly, plus your steering.
Properly driven these old trucks can handle fairly heavy loads. I overloaded mine recently to 24,200 lbs and it handled no differently than at 20,000. I don't advise doing this, however.
James
