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Just like any rust, you have to cut out the bad parts and weld new metal back in. It's a little more of a pita when you're dealing with multiple layers and drip rails, but the method is all the same.
The only thing that I would add to what Wayne posted is that you should gain an understanding of the design of the panel that you are repairing BEFORE you start the job. For instance, on a 56 cab the roof panel flange at the drip rail is the top part of a three layer pinch weld. The bottom layer is the upper section of the door jamb and sandwiched in the middle is the actual drip rail. These areas are prone to rust because the original seam sealer used in 1956 shrunk over the years and allowed moisture to seep into the seam. If you have rust-out in one area then the chances are that this is just the tip of the iceberg...the rust will creep along this seam even though you may not be able to see it now. Remove all old seam sealer and inspect the drip rail seam both from the outside and the inside of the cab. Remove and replace all rusted metal and use a quality seam sealer all along the drip rail. It's a PITA both worth the peace of mind knowing that you won't have to ever do it again.