My 59 3800 has 12 volts (original) and a floor starter, so it's not necessarily impossible to do. Also, your 6 volt starter will live a long time cranking on 12 volts, with little, if any concerns. It will just wing the engine over really fast!

Any good electrical rebuilder can convert your old starter to run on 12 volts by changing the field coils and armature, but 12-volt foot pedal starters are out there, factory original.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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