Oops my severe typo — I have a 76 tank, sender unit and the engine converted to EFI so I meant to ask about a 280Z gauge (not 260). But nevertheless thank you for the very thoughtful analysis most of which probably still applies. Will need to check where loads are relative to gauge. And when you say “try again to adjust the [fuel] gauge” do you mean one of the two pots on the back of the gauge, which pot and how hard should I attempt to crank it for I can’t afford to break the gauge!

And after some browsing, I found this image of the back of the gauge. The two holes circled have slots visible, but when I attempted to turn them there was severe resistance so I backed off. I also found this image which I assume depicts the slots visible in the holes, so there doesn’t seem much play here…

I can't find the more recent threads on adjusting the fuel gauge. I haven't done it myself. If I can find the time, I'll track down my spare gauge and see what I can set up to demonstrate.