Jim,

I-90 acrossed the western Gallatin Valley is gently rolling to flat. Whitehall Mt before Homestake Pass is 4,360ft. Then you climb Homestake which is 6239 (6350ft continental divide), then down into Butte, 5538ft.
I-90 to Missoula is more of the same like Bozeman to Whitehall.
The climb up north out of Missoula as others have said isn't anything worse than travelling from Billings to Big Timber Mt. or Sheridan Wyo, to Hardin Mt.
Have fun
Be sure to stop at St Ignacious and tour the chapel. nearby there is the Buffalo Preserve. In Polson, you might get in on the tail of the Flathead Cherry season.
The drive along Flathead Lake is beautiful, and once in the Kalispell/ Whitefish, Columbia Falls area, all the towns are quite quaint.