You'll want to get a new gasket for the sending unit housing. You want to make sure that the top of the tank is as clean as possible before you open it up. Make sure you are extremely careful with disconnecting the wiring. The plugs are very brittle by now. I cracked a couple of my connectors. Also, in addition to the sending unit itself, be prepared to replace the entire float assembly as it can wear a groove in the side of it.
To lower the tank, its easier with it as empty as possible. I used a large piece of wood to support the entire tank on a floor jack so I could loosen the straps and drop it. Removing the left rear tire make it easier to access everything. While you tank is out, check the metal supply lines along the frame for rust, leaks etc.
Good Luck!