[78SCRSMAN]

I'll be brief as I'm off to work for 12 or so hrs.

If I'm understanding you correctly, and I think I am, the panel meets the "b pillar?" as you have depicted in your diagram "A" (angled) It is spot welded (as is most everything on these cars) to the pillar. Simply drill them out to remove the panel. The welds between the rear 1/4 and the "panel" are easier to see and drill out from the wheelhouse side. Also, on my car, there is a joint at the bottom corner of the door that the factory leaded in. (see my pics) Heat up the area with a propane torch and the lead will melt away to reveal the joint. If you have more ?'s, I'll help as best I can.
Derek