Look up the shipping weight of a new truck bed for your year and model, that will tell you how much weight to add. If you are really a numbers guy, look up the build information sheet at the GM Heritage Center and see what the deflection rate of the rear springs are. Deflection rate is how much weight it takes to compress the springs 1". Use the spring rate with the shipping weight, and you should get a pretty close number to compress the springs. Hard woods are heavier then softer woods, so add or subtract depending on the bed wood you use. My '37 has red oak, I think the wood weighs as much or more then the steel did.
