For that corner with missing wood, I'd cut a strip of posterboard, or thin flexible cardboard (cereal box even) the width of the t-molding slot, and brad nail it temporarily over the gap to create the curve you want the final wood to look like. Now take small piece of wood, cover it with wax paper, and clamp it to one of the sides of your cab where the wood is missing. At this point, you have a makeshift tray.
Get yourself some bondo for 20$ at an autozone, or other car repair store, and fill the tray with bondo. That's a pretty big gap, so you're likely not going to be able to do it all in one sitting. The bondo will take some time to dry. I'd probably give it at least half a day to cure, though the outside will definately appear to be dry earlier. Don't worry about making it smooth when filling, just worry about filling your tray completely. Repeat until the gap is filled.
When it's filled, remove your makeshift tray, and sand smooth. At this point, there will still be some air bubble holes and other imperfections in your fill, caused by settling of the bondo, or maybe it seaped through the tray. No worries, now you don't really need your tray. You can slap additonal bondo on the low areas; make it higher then necessary and sand down. Repeat adding bondo/sanding down process until you have a new corner.
I've repaired areas of my cab with this technique, and it worked well. Search these forums for Bondo corner repair, and I'm sure you'll find pics of the process I described above. When you purchase your bondo and sandpaper, do grab the most expensive dust mask you can get.
-csa