Instead of going around Flathead Lake our Great Parks North map takes cyclists up Hwy. 83 through the Swan Valley, and uses mostly smaller county roads to reach Glacier NP. If you do want to cycle around the lake US 93 has shoulders and more traffic, as compared to Hwy. 35 which is narrower and in many places has no shoulder.
If you are visiting the office you can view this map.
After you leave the Grasslands National Park you should head farther north and not stay along the border. In eastern MT and western ND the Bakken oil field is booming and this field extends north into Canada. There will be high amounts of large truck traffic and gravel roads, the same as there is in the U.S. Google "Bakken oil field map" and you can see its range.
Here is the site for online cycling journals. You could search for Saskatchewan and Manitoba to see if anyone has cycled through those provinces with suggestions of routing.http://www.crazyguyonabike.com/