A friend rode the Trans Lab many years ago in later summer. I guess the flies about ate him alive which is why I am guessing you are going early? Sounds like a great trip.

Don't want to repeat what everyone else is saying but you would be buying a lot of bike if you got the Atlantis and you would be paying extra for the lugs and paint. I second the LHT, Trek, Urbane Cyclist, Koga and REI Safari. I like the LHT over the Trek, had both, because the crown fork provided more clearance and you will need it for the muddy places you will by riding and pushing yourself through. The Safari is aluminum thus harder to repair. Although frame failure is rare, it does happen but having said that, I wouldn't hesitate using it with a better set of wheels.

26" wheels would give you more bigger tire options although 700cs are catching up. Again, you will need the clearance even if you won't be using the biggest tire you can mount. You might also want to check out the Thorn Sherpa. It's their production touring/commuting bike and would serve you well. Also agree with putting more into the wheels and racks and not getting to crazy about the frame. My Bridgestone MB1 saw about 25,000 miles of touring before I sold it and a lot of it was on dirt/sand etc...

If you post your adventure, please let us know!