However... I made a big mistake by installing the window frames so soon.
The way the kit is engineered, you have to build the floor and the benches, than add the end walls to that assembly, then add the outer "skins" of the passenger compartment sides. As you can see in the photo in the last post above, the corners of the cabin walls have those mortise-and-tenon type joints. In most cases these joints are hidden by clever engineering, bit in the case of the main cabin walls, those joints are visible when the walls are installed. They have to be filed in with Bondo, then smoothed out, to hide the mortises and tenons.
What makes this sort of tricky is the fact that when you get to this point, the interior is already finished, and the glass on the end walls already installed. So I have to be careful when doing the Bondo and sanding everything smooth, I don't get too much dust and junk in the interior.
Also... I shouldn't have installed the window frames yet. You can see that I have to fill in the corner joints of the walls and sand them smooth while not scratching or sanding the window frames. It would have been easier if at this point the frames weren't already installed, but oh well...