There are two more things you can check. One is the bleed tube between the water pump housing and the block to make sure it's open. Next, try a different sender. I had to try three in my signature car before I found one that was accurate. I set up a portable stove with a pot of water, and immersed a thermostat, the sender, and a mercury thermometer and hooked the sender to the car with jumper wires. I had one sender that was indicating almost 100* just when the thermostat was opening and the thermometer showed the water was about 185*.

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'83 300D, 126K miles.