You'll need about 12" of height difference between your lowest level and the uppermost. I should have included this in my previous comment. Twelve inches will allow you to have staging tracks under your modular sections. I am planning only 9" between levels on my 2nd generation layout (a 14' by 28" around the wall).
Where did you find the original Armstrong layout plan?
I have a book from Model Railroader/Kalmbach Books titled "The Classis Layout Designs of John Armstrong".