The sending unit is a potentiometer (variable resistance) sort of like the adjustable light switches in your house to vary the light level. As the fuel level changes the arm moves changing the resistance. This resistance change changes the voltage to the fuel gauge which changes the needle position.
I've never used the 6 volt sender myself and someone here could probably provide more detail infomation on doing so.
Are you using a modern gauge set? If so I would obtain a sending unit from the gauge manufacturer that will work in your tank. The one in my '35 Coupe for the Classic Instruments gauge package has an adjustable depth to work with the original tank.
Hope that helps some and good luck with your project.