Can I use a rust inhibitor over existing rust on fuel lines and fuel tank?
 

Just bought a 98 E150. The frame is solid but I didn't realize how rusted some of the underbody is until I looked at it more closely at home. Instead of replacing the fuel lines, gas tank, and filler neck right away, is it possible to use a rust inhibitor such as stabil rust stopper or some other rust protectant oil to prevent this from getting any worse?

You have a lot of work before you. Two steel lines most important to REPLACE ARE THE BRAKE LINES AND FUEL LINES.

If a brake line develops a leak, pedal goes to floor, Big problem,
If fuel line to engine leaks , may not start or run and Hope you don't smoke near the vehicle.