I did the Northern Tier (N.T.) on an old all-terrain type (no front shocks/hardtail) fully loaded/self-contained and loved every bit of it. I did have the above mentioned end-bars mounted and did use them from with some frequency. The fat tires on your MTB'll give you the greatest freeeeeeeddddooooommmm
to get of the pavement whenever you want (watch for the spurs) to explore.
Btw, regarding down hill speeds, also mentioned above, at least on the western side of the N.T., the bike shop owners will alert you that there are now enough coyotes out there to be a problem at high speeds....they can not
hear you coming & will just wander out in front of you.... at high speed you have less control to avoid them than at low speed. Go slower and enjoy the view, I say
. Best wishes on a great trip.