Mark,
One thing to keep in mind is that your N&W J will operate on 0-54 curves. I have a SP 4449 and now NKP 765, both of which are about the same size as the J. If you can achieve 0-54 on your new layout, you'll at least be able to run all of your smaller stuff in addition to select larger locomotives. The overhang will be your biggest issue. It won't look prototypical, but it at least will fill the desire to run your big trains on the layout if you want. You have seen my layout in person. It is 7'x10'. I have achieved a decent amount in a small space using 0-54. The consolidation that I traded to you will look very nice on 0-54 as will any of your other shorter engines.
There are a lot of cool ideas posted so far. I like the thought of expanding your ceiling central to accommodate your biggest trains, but also keeping the minimum requirements for the main layout. Best of luck in selecting the winning design!
-Marc
