Mtnstream -
Have only fished occasionally in Montana so I can't give you a lot of help. Down here in Idaho, I consider August primetime for fishing small streams and small rivers. Missed it last year, because of the heat and drought, but this should be a good summer for both Idaho and Montana.
There is a really good website that you should check out -
www.bigskyfishing.com. This is the best site I've found for fly fishing information for Montana and is an excellent place to start your planning.
If you plan to venture into SE Idaho while in the area, let me know and I'll point out some good water for you to try.
John