You will have to purchase a replacement cab corner. then cut out the rust from the existing cab corner, grind the area clean, and then cut the replacement corner to either over lap the cut area, or to match up underneath the cut area. Weld or braze the new piece, and then flush grind the area. Light skim of body filler, and sand,prime and paint.
Eastwood makes a nice flanging tool so that even with the overlap, you will end up with a flush joint.
Now all this reminds me that I have 2 trucks that need some TLC.