Don't laugh, several people have told me I am crazy to go to so much trouble for an old truck, but the following works well for me. Wash it carefully using high pressure to get in all the little cracks, corners and inside the doors, let it dry for a day or so, then spray the underside of the truck, inside the doors, and anything that would rust with WD-40. Try to get the WD-40 in all the little places where rust might start. Obviously avoid the drivrshaft, transmission, etc. It "sticks" and stays on surfaces much better than you would think. If you regularly drive on dirt or gravel roads, this may not work since dirt may stick to the WD-40. I have an old 1975 Chevrolet pickup that I have owned since 1988. It was my grandfather's (really). It had been a farm truck used on dirt and gravel roads before I got the truck and was starting to get some rust. I have "WD-40'ed" the truck about every six months and have managed to avoid any serious rust.
My two cents worth.