Parklane you are correct. 1956 is the only year that the gauges received straight 12 volts un-interrupted by any type of reducer or regulator.
In fact, I was using a '56 gas guage with the original '56 sending unit, but now that I am being forced to use an aftermarket sender, I am going to re-install the original '55 guage with a constant voltage regulator wired ahead of the guage input terminal.