If you have any intentions of welding in the area of the bottom of the wiper box, I strongly suggest you pull everything out of the interior, the dash, the heater box, wiper parts, the plastic vent boxes, probably the wiring harness, and the carpet off the floor. You really need to get to a bare metal firewall. There is a lot of stuff in that area that will catch fire from the mig welder sparks that will be flying and some of the materials in the area will self feed a fire that may start. The entire seam along the bottom of the wiper box was filled (often very poorly) with a seam sealer from the factory. If you are welding in that area, the seam sealer really needs to be dug out. A fire starting to burn under a dash is in a difficult location to extinguish and would probably cause more damage the the water coming in would. Its not a quick weld job, I've done a few in the past.

While your checking stuff, it might be a really good idea to check the condition of the metal on the frame ear for the body mounts. The rubber mount will hold water and attract dust/dirt/sand and the grit of the dirt will wear away the metal under the mount. The result of that is there really is nothing keeping the cab on the frame, the metal will wear away to the size slightly smaller then the rubber mount. To check this, all you need to do is put a pry bar between the cab and the frame and see if you can pray the two apart. If its good, it shouldn't move, if its bad, the mount will pull right through the frame, any movement more the 1/8" is highly suspect. If your cab is making noises when the the body and frame flex, I would really check those cab mounts, all 4 on the cab, and the 2 at the radiator support.
That hasn't got anything to do with the water leak, other then someone mentioning body/frame flex. That Dodge truck frame and body were designed to flex, up to a point, its the extremes, or the deterioration of the original mounting system, that cause the problems. Gene