I had a similar problem with a '66 6 cylinder. Running down the road it was fine. Stop for a few minutes and it overheated. Tried almost everything. Removed thermostat, installed 7 bladed fan, built and installed a fan shroud, checked timing chain, and rodded out the 3-row radiator but no joy. I thought that the engine might be scaled in the water jacket and restricting coolant flow. So I took another engine I had and disassembled it completely, removed expansion plugs and cleaned the water jacket with a hacksaw blade. NO scaling remained. Did a complete overhaul and reassembled the engine. Installed it and overheating continued. Finally broke sown and installed an aluminum radiator and since I removed the thermostat, it will barely get the temp gauge into the normal range now. I think I found the radiator on EBay for around $110.