Cab corners look good!
I have plenty of welding experience & equipment, but very little skill with sheet metal shaping. The reason I find it surprising that no one offers the panel commercially is 1)In this part of the country, that spot is one of the first to see rust holes. 2)Inner and outer roof panels are produced and easily available. I couldn't imagine replacing the two roof panels and trying to "reuse" this little panel.
Same panel is also junk on the parts truck.
All vehicles have their trouble spots. I have done roof repairs and drip rail fabrications on two different 55 Chevy wagons, where the only explaination possible was condensation from the inside caused the rust that occurred from the inside out. I'd say the same situation caused the damage in your truck(s). Based on this being a typical trouble spot, if you did find a donor cab somewhere with a good panel, likely the rest would be too nice to cut up. If I had a "sample" truck to work from here locally, I'd make one for you. From the looks of your cab corner repair, you should have no problems installing a replacement.
