In may cases when converting to 12 volts people jsut let 12 volts run
through all the gauges without changing anything.  I'd be less worried about
the sending unit as I would the gauge.  Don't forget all of the other gauges
will be affectted as well.  You could stepdown the voltage back to six for
the instrument, but that would be a lot of work because you would need to
completely isolate those circuits.  Personally, I would run them as is.
(Actually if I was doing a "restoration" I would maintain the 6 volt system)
Steve G.

The way I did it in my 1940 Ford was to go to my local electronics
shop and get the largest volume pot I could find (Not size but
Wattage). Supply the 12v across the pot and send it to ground through
a small light bulb, paint or hide the bulb as nessary. I myself used
them in locations behind my dash board. Use a good meter and adjust
the output of the variable connection to 6v this is ease to do.

What I did with my 50 Merc was to install a 6/12 battery.  With this
everything but the starter runs on 6 volts.  The starter runs on 12 volts.
This battery is two 6 volt batteries in one case hooked up through a
series-parallel switch to handle the separate needs.  Charging is through
your standard 6 volt generator.  You can have the starter rewound to 12 volt
if you like but I didn't and never had a problem.  The short starter run
duration  with the 12 volt didn't seem to hurt the starter.  It sure spins
those old motors over, especially in the winter.  Look in any Hemmings book
for vendors.