Can't wait to see your progress. You are doing it right. Lots of thinking before making sawdust. When I saw the photo of the Davis engine house I said to myself, hey I have seen that before. I kitbashed 2 Lionel loco sheds by removing one panel from each side of one shed and gluing those to each side of the other shed and then making 2 new ends with 2 portals. The doors were easy to make from skinny sticks. I traced the openings from the donor opening. It is a pretty big bldg so it takes up space. But I find the proportions and design appealing.