Another overheating situation 63 Slant six

Had a very similar problem years back. Pulling the cap off the flow looked good no real calcium build up. It drove me crazy. I finally took the rad to a shop. Down below where I couldn’t see was major league clogged. When he rodded it out the rad crumbled in his hands.
As already said check the pump, also could be a hose collapsing , and if you’re losing water Before it overheats could be a blown head gasket. Check to see if the oil appears milk looking.