Floor pans are one of the easiest body panels to replace, mainly since they are covered so they don't need to look perfect. You can either just patch the rusted spots by cutting bad chunks and welding in prices of sheet metal, or cut out and replace the whole floor pans with aftermarket ones. Even if you don't know how to weld it wouldn't be too expensive to have a body shop do the work. I would never give up on a truck in great shape over some floor pans. But you do need to fix the issue that made them rust out, obviously they were holding water from a leak somwhere.