The oem '55, '56, '57 gauges and senders are each a one-year-only internal design.
The best chance you've got of having the Fuel level read correctly is to have a year matched gauge & sender pair.
Either oem or a reproduction pair, but not a mix.
The original '55 King-Seely type gauge & sender work on current flow and heat generated thru an on/off switched circuit, not variable resistance or voltage.
If you have a matched pair of those in good working condition they usually aren't voltage or polarity sensitive.
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You do not need to change any switches or gauges. The fuel and temp gauges will work on 6 or 12 V and are not polarity sensitive."
https://www.ctci.org/battery-for-6v-to-12v-conversion/