Warren -
I don't want to rain on your parade, but I believe you should take a close look at the river conditions in the west this year, if you have not already done so, and particularly as you get closer in time to your departure date.
I can't speak for the Henry's and South Fork, but I live on the Yellowstone during the summer, where I am right now, and I think it is likely there will be little to no water flow here by August, and a good chance that there will be "hoot owl" fishing restrictions on the Yellowstone, and other area rivers, then, which close them down during the afternoon due to high water temperatures.
Many of our Montana area river flows are presently just half of the normal flow for this time of the year due to an abnormally low snowpack, and warm weather. There are already a number of fires burning in the northwest US and Canada, our air here is smoke filled, and we really haven't even begun the typical year's fire season.
Late August is normally one of the most difficult times for fishing here during the best of times, in my opinion, and I'll include the Henry's Fork, South Fork, Missouri, and other area rivers, all of which I have fished many times when I say that. However, this year is not shaping up to be one of the best of times.
I would suggest you might want have some flexibility in your plans, and not get too locked into specific rivers and locations this far in advance. There might be some fishing opportunities on some, but not all, of the rivers you mentioned come late August.
Good luck.
John