I know-knew, but for the life of me, I can't remember, can you run a 6 volt gas gauge through 12 volts and have it work accurately? Got a friend, (yes, I have friends), with a 47 chev coupe, older hot rod, probably built 30 years ago, looks to have been a cream puff back then. Looks like a nice original with mustang suspension, small block-T350, and ten bolt. Stored for a long time, woken up, put back on the road last year, but the gas gauge doesn't read correctly. All the rolling stock looks like original stuff, just looks like the drivetrain was replaced.
I saved this from a thread here. Let me know if this doesn't work for you guys, and I will try something else.
Ford used a voltage reducer under the dash thru the 60's and early 70"s. Simple bi-metallic unit that works well. Cheap and available.
I've used Those ford voltage reducers a few times but find they are unreliable when I use them. So I put the runtz reducers in. And they work perfectly every time.
Actually not a resistor, but a voltage regulator. Here's an older thread on how to make them. It's the same as the .pdf file that @BJR posted: https://www.jalopyjournal.com/forum/threads/how-to-build-a-voltage-regulator-for-3.448589/ Or you can buy them from a number of hot rod suppliers. They are advertised as a Runtz Votage Reducer.
I followed the instructions in the pdf I posted. I ordered the parts from the net and soldered them together. Used them on my Buick and they work perfect. Cost under $5 each to make.