I took a similar route last summer in the opposite direction. No problems with the road, as has been said; it's a nice, easy two-lane highway. I rather enjoyed the Havre Underground tour, and did not stop at but heard very good things about the buffalo jump site in Havre also; it's said to be one of the very best preserved such sites.
If you have the time and haven't been there before, I would suggest a day or two at Theodore Roosevelt NP; it's beautiful badlands scenery with plenty of opportunity to see prairie dogs and bison pretty close at hand, among other critters.