Around the gutter
Middle door pillars
Back of the cab (cab corners)
The floor and the roof seem to get hit the hardest with rust on the crews. The 68-72 trucks also can get rust around the seams. The weld seam that runs under the back of the front seat is the main cab splice for these trucks. It should be sanded and covered with a rust paint. I would think that sanding the underside of it as well would be good. If you find any holes, mig weld them, because that metal is not the best to cut out and replace.
Anywhere that Ford welded and cut to make our cabs has a potential issue.
Also, save yourself money, and vacuum out the inside of each door till itâ€™s clean. The small drain holes at the base of the doors work great for creating a nice damp rust breeding environment that can eat your door skins, and cost $100's to fix. Just vacuum it once a year or so, and she will be happy. If you start seeing rust in there, you may want to remove the glass and hit it with some rust neutralizing spray and krylon it. If you donâ€™t remove the glass, you could get spay on the glass.. and that wouldnâ€™t be pretty.
Almost anything on the truck body can be cut out and replaced with a good mig welder and grinder. I have a Lincoln 110V mig that works great for mild steal and sheet metal. Just take your time, and practice on sacrificial metal that is near the same type and thickness of your intended victim.. ur I mean target.
When you weld, tack the metal in place using small welds 8 inches apart. Donâ€™t try to weld the entire thing at once, or the metal will warp, and then you have hours to get it back in shape. Small welds, far apart, does a much nicer job then being in a rush.