I noticed, while building the first three of my 12 stalls, that a very small gap needs to be left between each section in order not to get such a large gap at the end. The gap between sections looks like nothing more than a shadow.
Looking at your pictures, I notice the rear walls are butted up against, instead of on top of, the concrete floor/base. Those “tabs” on the rear of each stall’s base are for the rear walls to “index” against. The notch on each side of the inner surface of the rear wall “wraps” around half of the rear support for the roof truss.
Why didn’t you write the instructions!! I built a pilaster system and it will work. Would have been nice if Walthers made that clear. Will watch for that on the Modern kits. It is glued with PRO WELD so no way you can take it apart.