I've been riding 650b x 42 for 2 years now on my rando bike. The 650b's deal fairly well with the terrible road surfaces around here, which was the main factor for going that direction.

Main issue you might want to consider: tire availability when on your transcontinental endeavors. I don't know how much one can count on finding replacements in the middle of potentially nowhere.

I did take the bike on a gravel 'race' and PBP this summer. Bike was great in both cases.