Paul, I have a restored '36 Chevy pickup. before I restored it I ran it for about 50,000 miles as a sbc powered rat rod with a 12 volt electrical system. The stock gas gauge and sender worked perfectly on 12 volts with no voltage reducer required. The gauge has opposing coils that pull the needle towards the coil that gets the most current so 12 vs 6 volts made no difference. If your gauge is the same all you need to change is the light bulb.
To me part of the fun of this hobby is finding the most cost effective way to get the job done. I hope this helps.
