Get as much of the rust out of the bed as possible. Throw some rust converter on what is left. Let the converter do it's thing. Use the cured converter as a primer. Now throw on some bedliner of your choice. Heck even some aersol can undercoating might work. Paint work on the outside is optional. Plywood floor liner is a must. Getting rid of & stopping the rust's progression is the main concern here.
THE WAY I WOULD DO IT IMO.
THE WAY I WOULD DO IT IMO.