The Disuster
The instrument cluster grounds to the dash frame with its mounting screws. This was prob ok when the car was new. But almost 50 years later I am sure grounding point degradation here has taken place. Most people will fashion up a separate ground wire from the metal cluster housing and run it to a good body ground. I recommend the lower dash frame to body mounting bolt, and a good ring terminal.
The steering column ground wire is for grounding the column to the body as a return for the horn wire.