One of the best things you can do to a old fuel tank is have it boiled out at a radator shop then coated on the inside. That is the cheapest insurance you can have done. On the out side you can spray it with a cold galvinize spray then use a good mailbox silver on top if it's on the outside of the truck. If you have it on the inside you can get the treated tar type paper orginally used to cover the tank.
62Blue
Don