For the purpose of figuring the body capacity it is a common industry practice to allow 3000 lbs per cu.yd. Some commodities will weigh a lot more but dry gravel, asphalt, dry earth and many others all weight a little under 3000 per yd. The hoist is sized to the body capacity so it will probably lift more than it will hold unless it's full of cannon balls or concrete. Hoist capacity is calculated by class, i.e., 20,30,40, etc. which is the ammount of torque around the rear hinge. Common hoists in 1963 for your chassis would have been a class 30 rated at about 10-12 tons depending on body length. A common single axle dump of that time would have ab 8'x6'-6" ID body with 24" sides/30" ends and rated at 4/5 cu. yds.
