I once rode the route Carla mentions, but in the opposite direction. It's nice. Camped at a state park in Big Fork and then Lake Inez or Lake Alva U.S.F.S. campground (Can't remember which. The one that has water) before heading to Missoula.
If you do go into Canada after Glacier N/P., I highly recommend going to Waterton Village for a rest day if desired. The towne campsite is in a dramatic setting along the shore of the lake. Plenty of services in town, and there is a combination boat ride/hike you can do. Just don't underestimate the difficulty of Chief Mountain Highway, especially if you do that leg the day after Logan Pass. And both times I rode the spur off of PR 5 into Waterton I had a pretty stiff headwind. On the flip side, the tailwind from on PR 5 through Cardston to Magrath was awesome.