There is really only one "mountain" between Yellowstone and Glacier on US 89 (Little Belt Mt and Kings Hill grade)
In fact most of the road, after the Little Belts, is flat driving. Actually that's because from Great Falls on you follow I-15 for much of the drive.
As noted, traveling up the west side of the divide (I-90 to Missoula, then north) is much more scenic.
In order to get to roads that would not be "fun" (ie tough) you would have to drive up the back-roads. Even then they are not bad
Ya, I've probably drive most of every road going from one to the other.