You should be aware that this is a lower than normal snowpack year for much of MT, and rivers could become quite low by August, including the possibility of some stream closures if water temperatures get too high.
I would say pick your outfitter to line up a guide, but ask what would happen in the event of too low and/or too warm water conditions. I think some guides might move from the Missoula area to places like the Missouri River if that should happen; that could be too far for you to commute.
John