On the early gauges (before the voltage stabilizer was added) it is imperative that the gauge itself have a good ground connection. <span style="text-decoration: line-through">If the gauge looses its ground it will read full. </span>Start by running a dedicated ground wire for the gauge to see if that restores normal operation.
EDIT: The gauge will not work if it is missing its ground.
The next thing that will make the early gauge read full is if the green/black wire is disconnected (at the sending unit or at the gauge) OR if the sending unit has lost its earth connection.
Several years ago I wrote a letter to a guy in the U.K. who was working with the early gauges. I saved that letter in PDF format for reference. There may be some information in the PDF that will help you troubleshoot this problem:
https://home.mindspring.com/~purlawson/files/OldFuelGauge.pdf
You may also want to Google for "MGA Guru" and visit Barney Gaylord's web site. He has a section devoted to troubleshooting, testing, and calibrating the early gauges.