500 gallons at 8 pounds per gallon would be 4,000 pounds, well within what I'd expect to be reasonable on a 6400, provided that tank didn't weigh any more than another 4000 pounds...
Its been 25 years since I worked on a farm, so I don't remember all the details, like length of the beds, and height of the side boards, so I could be fuzzy on some details, but I'm pretty sure loads bigger than 16,000 would have been real rare on trucks the same class as your 6400.
