My 5.9 Magnum Build - Junk Yard Mule

That chalky black appearance on the internal surfaces of the block is usually an indicator of poor maintenance, particularly lack of oil changes. I believe the oil gets really acidic and loosens the bond between the bearing material and the steel backing. The cam bearing in your picture is not worn, it is flaking away. If it was mine, I would replace all of bearings, including rods and mains, and carefully inspect the crankshaft bearing surfaces.
Next, you will want to check all the pistons to see if the oil drain holes inside the ring lands are blocked with carbon. This is common on Magnums with lack of maintenance, and maybe a leaking plenum gasket contributes as well.
It will still be a good core to build even if re-using the old pistons.