If you are going to St Anthony, why not just get off of I90 at Belgrade and take 191 up to Highway 20 at West Yellowstone? That would be a much more direct route. Then you could cut back over to 93 when you leave St Anthony.
We came home that way from SD in May. A pretty drive. Just laugh at the GPS when it tells you to turn left at Jucribbit Road. It is actually Jackrabbit! Ours also kept trying to get us to go further west than 191, as well as trying to send us through Bozeman. Fortunately we were following the paper map and just checking the GPS.
