Sounds like a great adventure, having said that did you include all your things you plan to take with you in your fuel consumption data? If you add an extra fuel tank you will loose some MPG. Also with putting stuff on your roof, major loss think 2-3 MPG. If you build your drawers make them light, anything added to the vehicle will add to the end result, lost MPG.
As for taking the long way, I think it would be awesome to do. If I had the time and gumption it would be a trip that I would love to take.
Good luck and let us know if you need help. I don't know much about 40's but the others do and one way or another somebody can and will help you.