I’ve flown that route twice. As I recall, not a lot of private strips around or near I-90. I generally flew between 9500 - 11500’. I-90 will give you the lowest overall altitudes if you roughly parallel it although at some points you may want to be nearly on top of it, depending on how high you can or want to fly. Once you get to Missoula, you will need to be concerned with Lookout Pass coming into Idaho. You can get the weather at Lookout, but it can be the case that Lookout Pass may show MVFR or lower with VFR on both sides. It does block a lot of weather depending on the time of the year. I’ve heard of some folks cutting north from Bozeman to Great Falls and then from Great Falls to Glacier and then onto Bonners Ferry in Idaho. You will still need to get somewhere around 9500’ for periods of that route and again, not a lot of airstrips in that part of Montana. Once in Bonners Ferry (which by the way has a great FBO and the cheapest gas in Idaho) it’s south down the valley to Coeur d’Alene and then west to Spokane. If you come over Lookout Pass, then it’s a fairly straight shot into Spokane with descending terrain. Flying around Coeur d’Alene lake is very pretty and well worth the time. Keep your head on a swivel. Fly the right side of the Lake heading south or north. We had a mid-air collision last year over the Lake when pilots ignore this rule and 6 died.
From Spokane to the Cascades, it’s relatively flat, particularly if you parallel I-90. It’s a very do-able trip. Fly early and get down early. Read about and become familiar with mountain flying including mountain waves, etc. Plan lots of outs and don’t push the weather. If you have to overnight, no big deal. Make sure you have some survival items - water, food, etc. and know that cell coverage can be sparse in many areas. If you don’t have one, consider purchasing a Spot, Garmin In-Reach or something similar. I do use a Spot when flying in Idaho, Montana and other places in the West. Those are mighty big rocks you will be flying over and around.
