With the float rising and falling in the tank it changes the resistance to the gauge. So when the float is high (full tank) there is low resistance and the gauge should show Full. You can test this with the sender out of the tank but you must ground the unit. Make sure your tank has a good ground.(use a second ground to check.) Normally the sender unit is matched with the gauge as a set or pair when new ,so if you have the correct replacement sender and you have made sure your gauge has a good ground connection ( add a second one to prove the ground connection from the gauge to battery negative.) the only thing left is a stuffed gauge.
the rusted fuel tank straps could have prevented a good earth as will painted metal. Clean metal gives a good earth.