Fuel sender

I have a 1952 Plymouth Cranbrook. The car has been converted to 12V

I thought the gas gauge was working. When I fill the tank it goes all the way over past the full mark. After it gets down a few gallons it just drops to E

Does it matter if 12 or 6 volts are going to the gauge?

Before I spend the money on a new sender should I try a dropping resistor in the gauge?

Thanks all.