East to Cardston and hang a right. When I did the Northern Tier route back in ‘99 ACA used Del Bonita south of Magrath on the way to Cut Bank. What a lonely 70+ miles that was. I remember one little store just north of the border. And the headwind was pretty evil. My riding partner and I finally stumbled into a cafe in Cut Bank super loopy. The other patrons all turned to look at us.
Expect a stiff headwind coming down 89 to St. Mary. At least you won’t have to do the climbing on Chief Mountain Highway. That is not an easy day.
Depending where the hiker/biker closure is, it might be worth setting up shop at either St. Mary or Rising Sun and riding as far up as you can then back down.