Your old friend and your own experience pretty much tells the story. The guage will work equally well regardless of the voltage. I know of at least one fellow bolter that has been running his '49 on 12V since the 70's with no problems.

To troubleshoot your guage misread problem, take the sender wire off at the tank and let it dangle in the air, turn the key on, the guage should read full. Now with the key still on, ground that sender wire to a known good ground, the guage should swing to empty. If this test proves the guage to be good, the problem with your guage reading full all the time is either an open sending unit or a bad ground at the sender tank flange. Yoou may have to run a seperate ground lead from the sender flange to a good ground.