Re: Ft. Smith to Craig
The big questions would be how much time do you have and what month of summer? Because from here to there are a lot of options, but the Yellowstone is the only one "along" I90. That being said the "Stone" is right there From Livingstone to Columbus (after that it gets kinda murkey) though is a tough wade in the summer months best to be floated.
My neck of the woods (Columbus exit) has the Stillwater, Rosebuds (Laurel exit) Rock Creek. - All which are great for wading depending on when runoff ends.
You have Three Forks and Four Corners too with the Jefferson, Madison, Gallatin Right there off of I90. I know Gallatin can have some great wade fishing might be crowded due to easy access though.