I would do what oss said. Very nice route.

Ive been on 85 outta bel fouche for a ways and its not bad. You can take 12 in ND to miles city MT. From Baker MT to Miles City MT is really nice and easily do 80+. I passed 6 houses and 8 cars in that whole 88 mile stretch. ??? Spent the night in Baker MT and liked the little mom and pop joint on the east side of town. Park on concrete at your door.
oss is right about doing the Bighorns. Well worth it. Rapid to Cody WY is do-able in a day. I left Sturgis on 3 hrs sleep and severly hung over and did it in a day.

Be hard to find a room in Cody tho, so you may want to call ahead. I stayed a block away from Bill Codys Erma hotel. Had supper there and a few beers. Neat watching the deer walk around downtown. Nice town.

After you hit the bighorns, Cheif Joseph HWY, and Beartooth, you can shoot right up to Glacier.