Changing the tank sender is a better idea than using a parallel resistor. The parallel resistor solution is very inaccurate when the tank is not full or empty.
Also, with the quality of today's modern electronics I would consider getting a voltage regulator rather than a resistor to convert 12V to 6V. Today's 12V systems typically run at 14.5V. It would take a very interesting resistor to maintain a constant voltage with a 2.5V range on the supply.
DC - DC Voltage Regulator.