I went back and read some more, but don't see where you had your radiator professionally cleaned. I saw where you had run some water through it.
There could be enough passages open that water flows through, but not enough open to take the heat away as it should. Coolant moves through it much faster when going down the road than a water hose setup can duplicate.
Does this engine run hot when idling? Or only under a load running down the road?
On the 216 I don't think you can install the head gasket wrong. If it were bad and compression gasses were getting into the coolant system you would have bubbles show up while it was running when you had the thermostat out.
I think you need to go take a closer look at the radiator. Take it to a shop and have them open it up.
