How much weight on ledge in Ranger bed?

I have no direct experience or specifications to provide. But I have observed that the sides tend to bow out at the top in older trucks, especially those with the tailgate removed. So I would fabricate some rigid support at the back and middle between the top of the two sides before you get deflection.

Good idea. You might consider making rails with a vertical "peg" of some sort that would fit into the stake pockets to keep the sides from bowing outward. As far as overall weight is concerned I would worry too much about that as to how much stress you're putting onto the bolts that hold the bed to the frame when you go around corners with a lot of weight. How high do you stack this stuff anyway?

Just when I figured out how to get the toothpaste back in the tube they come out with this stripped stuff!