Lots of places to see along the Lewis and Clark Trail. Obviously, they spent a lot of time in some pretty rugged territory, but they did "blaze the trail".
Perhaps you could just look at some of the places that they stopped along the way. Lot's of great scenery!
https://www.nps.gov/lecl/learn/historyc ... -route.htm
That isn't quite a DUE NORTH route for you, but close.
St Louis is officially "the gateway" to the Lewis and Clark trail, but there are historical places east of that (may surprise you)
Council Bluffs Iowa has a "Western Historic Trails Center" - where they show the routes of four major trails involved in "western expansion"..
As mentioned, MT Rushmore is really worth seeing (as long as you don't try to travel during "bike week"...
I did this and went out of my way a little bit to see "Chimney Rock" in Nebraska and "Devil's Tower" in Wyoming. (Both of these are quite a ways from interstates, but nice to see)
Montana has a site commemorating Sacajawea's son (that was born on the trip westward with Lewis and Clark) Pompey's Pillar - which isn't all that far from Little Bighorn.
FYI - Montana and Idaho have some SERIOUS HILLS on the interstate (and elsewhere) - so I would doublecheck brakes and other equipment that can be affected by long (but beautiful) hills and mountains!!
Praying for a safe trip for you!!!
PS - Edit - something I forgot. If you find yourself going anywhere near Cincinnati, OH - I haven't been here yet (but was born close to this place several decades ago) the Creation Museum or, even better, The Ark Adventure. Both of these are in northern Kentucky - not far from interstate 75. I have a friend that just returned from a LONG motorcycle trip, and he was just IN AWE of the Ark Adventure. The highlight of his trip!!!
Nearby (relatively) across the river from Louisville, there is Clarksburg, Indiana. Named after George Rogers Clark - who, by the way, was an uncle to William Clark (of the Lewis and Clark expedition) Clarksburg has, what I think is the MOST Recent "commemorative site" for Lewis and Clark. I stopped in there a couple years ago when I heard this - and some vandals had just burned the historic cabin. Still, the view back across the Ohio River from there is amazing!!