A few suggestions. At Henrys lake ride up 87 to the ID-MT border and ride the divide to red rock pass. This is a beautiful stretch of the divide. And when you ride by the red rock lakes, go on the south side. The north side has miserable sand dunes. Like riding through 2 feet of flour.
Not necessary but if you want more back roads in MT you can cut from painted rocks lake to the nez perce road, as a slower but shorter route. Later you could look at cutting accross Dorshack resevoir at Dent or Grandad bridges. Also you can turn east at the town of Clarkia and ride over freezeout and eventually come out at Avery. Different options.
Can provide more details if interested.