1955 went to 12 volts so the eBay starter is misidentified.
Yes you can use a 6 volt starter on 12 volts.
There is no good reason to convert to 12 volts unless you want a 12 stereo or cel charger...
To convert to 12 volts you would need to change the bulbs, gen/alt, heater motor, and put a dropping resistor on the gas gauge.
