Or you could take Highway 200 out of Missoula ( the east side) over to Helena.
Stop for a few minutes in Lincoln at the Ranger Station to see a huge stuffed Grizzly Bear. Was hit by a truck a few years ago. You would also be going over Rogers Pass, near the spot where the coldest temp was ever recorded in the lower 48 (-70F).
SCAN Web 6.0 - Camera
If you didn't want to see the bear you could go via 200 and drop down to 12.
When in Montana have some 20 dollar bills available, you can pay your speeding tickets on site with Officer Friendly.
Or at least that used to be the going rate.
bob