My brother came out to work on the bus with me yesterday. The fiberglass cap (?) that covers the whole rear end kicked our butts. We got the window out and took the hood/door off, but I don't think we got any of the rivets out. We drilled many of them but then couldn't get the remaining body of the rivet to push out through the back. It is glued all the way around too, and I haven't figured out how I'll break that without damaging the fiberglass. So we let that be for now and moved to something else: the rub rail between the doors on the right side. Got that off and then we were so excited we pushed on and removed the front three lower side panels.
Check out that rib between the two windows -- it is filled in! Here is a close-up:
The hat channel has a piece of.. something.. welded inside it. I don't know yet whether it's a closed tube, or an open C or U shape. In any case I'm now even more happy to have picked the "make more hat channel" route -- if not for that, I'd have a stack of square tube and a broken plan for using it.
I guess these might be done this way because of the large windows -- the bus has only half its ribs actually reaching the floor, and each of these has to carry its own weight plus half the weight on the ribs above the windows too. I'll stick to the original plan of extending the hat channel only, and not extend this surprise inner section. Since I'll be adding new full-height ribs in what used to be the middle of most of the windows, I think it should be fine.