This is a great way to do it but unless it’s some special vehicle or has sentimental value to the owner it makes no sense to do this much work. I would just cut my losses, drive it as is or sell it and get a clean rust free vehicle. You CANT reverse 20-30 years of NJ salt damage by wire wheeling and spraying chemicals here and there. 100s are not that hard to find and the high mileage cars are not even expensive.Without cleaning and neutralizing the oxidation, you will be wasting your time and money. Do a little internet searching for the rust kill product that will work best for your budget and work area. Any type with an acid base will clean the metal, any "converter" type will stop the oxidation and neutralize the chemical reaction that causes the destruction. But... YOU NEED TO CLEAN and PREP the metal before anything else... Without seeing the rest of the truck, no one here can offer an opion that is worth the time to type. Only YOU can decide if the truck is worth putting in the effort to correct what has already occurred. Based on just the very limited picture of what you have presented, (IF THE ROCKERS, FLOOR, FRAME RAILS and LOWER QUARTER PANELS are not rotted away! Image 5 and 7 give me pause for concern) my first inspection would be to hit the frame with a pointed body hammer to see if it breaks through the frame. If the frame falls apart, no need to read further! I believe I see in image 5, a piece of flat stock has already been welded to facilitate some type of repair previously... image 7 appears to show a perforation of the side of the frame rail.
I would strip the underside of the truck entirely... exhaust, suspension, driveshaft, cross members, bumper covers and anything attached to the underside of the vehicle, (about 4 hours) needle gun the frame, vacuum out the frame rails after, air-blow the frame inner and outer, grind/sand the exposed oxidation (36 to 80 grit discs) spray the rust kill/converter of your choice (garden sprayer), brush it into the hard to reach places, let it set per product specifications, scuff pad, prime and paint. I would then Woolwax the complete inner frame and all exposed areas underneath to include the rockers and floor boards. BUT...