I haven't yet worked out how to attach a photo to a reply so in response to Mikkel's comment I attach a photo of the inside of the first almost completed coach.
Unfortunately my camera and lens is better at landscapes than close ups but I hope this shows how I tackled the fitting of the body to the chassis. The folded tabs on the ends are no good as using these means that the ends, and consequently the sides, sit on top of the floor. I cut these off and made my own tabs out of brass sheet bent to an L shape. The shorter side is held against the ends with the longer side resting on the floor. When the position is correct, i.e. the ends and sides are just overlapping the floor, the shorter side of the L is soldered to the end. In retrospect the angle of the "L" could be a little more than 90 degrees to accomodate the slight tumblehome on the ends. The steps on one end don't help in getting a good fit between the new bracket and the end but I filled the gap with some scrap brass.
Having accomplished this I then turned my attention to the holes for the screws located at each end. These had already been drilled through the floor so it was a simple matter to mark the position of the holes on the bracket, drill through the bracket, and firmly solder the 8BA nuts on the top of the bracket. A good tip in the instructions was to use cocktail sticks to locate the nuts through the holes which avoids soldering up the threads.
I hope the photo and the explanation clarifies. I hope the finished coaches justify the huge amount of time and expense incurred in contrsucting them.