I'll throw a shot at it.
Disconnect your wire to the sender. Take resistance readings at the sender with a multimeter starting with engine cold, and periodically as it heats. Try to see where the temp vs resistance chart goes off the rails, or if it tracks as it's supposed to.
At 70*, resistance should run between 600-700 ohms. As temp goes up, resistance goes down, where you might have 50 or less ohms resistance at 250*.
Some have used variable resistors to check the proper operation of the gauge.
I saw something on team Chevelle where 68 seems to be an odd year. Maybe your sender is not right for your gauges.
https://www.chevelles.com/threads/co...-unit.1084088/