Your C-series may be different than my B-series truck, but it looks similar. Your drawing is close, but there is another piece of sheet metal between the hinge and the threaded plate. As I remember it, the hinge slides into a pocket in the A pillar and the threaded plate is captured in front of that pocket. I recall that it can float slightly, but is caged in place when the bolts are removed. As mentioned earlier, try some localized heat on the bolts to break the rust bond. I’ve had luck in similar situations by heating the head of the bolt to cherry red, letting it cool, sometimes repeating that process, then try loosening again. The heat-cool cycles will weaken the rust bond. Of course that would damage the surrounding paint if you are worried about that. Otherwise just rattle away with an impact gun to see if they’ll come loose. Switch the gun between loosen and tighten as you rattle on it.