how to fix speedo

You might be better off in the long run to have yours repaired/rebuilt. After 50+ years and a 100K miles or more, the bushings, etc.., are surely worn. Directional forces coming from the speedo cable is what generates/increases wear.
In 1968 the cable end changed to a nylon sleeve which aligns the cable to the instrument input much better than the collector nut found in previous years ( like yours ). Anyway...
With rebuilt or working used instrument, clean the inside of the input, make sure the cable end is slip fit and cable is aligned to the instrument as straight as possible, snug that collector nut.
All too often a new cable is poorly routed, the square end is crammed into a port filled with hardened grease and debris. Then the nut is left less than finger tight. This will kill the instrument.