Why 10.7 volt input at ‘71 Duster instrument panel?

Clicking in speedometer can be a couple of things... Cable applying pressure in any direction can move parts inside the head, especially where the tiny bronze bushings are worn. Another is the odometer spool. Either should result in a needle twitch or jump. When you get the panel out, lay it so the weight of the panel is against the input port/drive spindle of speedo. If the needle lifts more than 5 mph there is excess wear in the bushings. With the speedo out on the workbench, look closely at the drum shaped part for rub marks where one part is touching the other. If you can't find anything wrong with the head, check that the input port is/was properly cleaned. Route the drive cable to it a straight as possible.