This turned out to be an easier fix than expected, problem was the little constant voltage regulator that bolts on the instrument gauge cluster backside.
About 1.5 years ago I installed one of the new solid state regulators that apparently was not so.....solid. Never really considered this might be the issue because all the other gauges worked fine, but since the cluster was out to check connections decided to swap in an NOS regulator just in case and bingo, fuel gauge now works perfectly!