There is a few ways you can go about this. One is the most expensive and that is to pull the body off the chassis, put on a rotissery (sp), get it sand blasted and use automotive grade paint and paint it. Another way is to build a stand out of wood, metal, even just jack stands but make sure it's secure. Pull the body off with the help of a few friends, set it on the stands, crawl under and using a blaster or a wire wheel or whatever you want, get what you can off, seal with primer, use a basic single stage paint. Or if there is some rust you can POR15 the underside and leave it. The POR15 will seal the rust from spreading, it is gloss black, you can brush it on and it will smooth out slick.
Whatever you do, the best is to pull the body off the chassis and clean it up best you can.