Cutting out rust and welding in patches will not last very long unless you are able to prime and paint it from the inside. An entire new panel (bedside, fender, door, etc..) is the only long term solution. When you weld a patch in you burn off the paint on the inside of the panel. If it rusted through the panel from the inside to begin with you are in for a treat with bare metal on the inside. I have done close to ten trucks repairing rust. Entirely new panels and seam sealer are your best friends.