Cruise control will not work .

How does it know your speed? I would not think there was a sensor in the speedo head that early monitoring the needle? (But my Dad borrowed a Riviera with an overspeed alarm on the speedometer that you could dial in) First thing I would check is cruise control vacuum servo with a mighty vac after you lube the speedo cable with graphite. you can get it powdered in a tube for locks. I dont think grease is the prefered call out for a speedo cable. Let us know what you find.

PS>> there is no sensor in the speedometer head, the sensor is in the CC servo itself as there is a speedo cable going in, and a speedo cable going out to speedometer. I suppose it could be completely mechanical but it would take a mechanical switch/cable to manipulate vacuum signal.