The first thing I would check is the ground for the sending unit. The ground wire is attached near the passenger side tank strap mount (close to the differential). Remove the screw and clean up all the contacts (trunk floor, screw and wire connector). Make sure the screw is tight when re-assembling. Test- if there is no change then the sending unit rheoshat may be bad. Check it with an ohm meter as above.
See my '68 RS/ZZ4