I have cycled close to there (Bluff/Chinle), but not on 491. Spot checking with the google guy shows smooth pavement and wide shoulders. 42 miles with a downhill slant. You should be just fine. Take plenty of water as there's nothing out there. Hopefully you hit it on a good wind day, as the wind generally comes from the south. Earlier will be better, temperature and wind wise. Probably it will be a whole bunch of nothing. I find that to be excellent riding! Have fun.