I will not attempt to give you any advise on fishing the Bitterroot as I have not been over there for many years. (To many people anymore in that area)
But I just wanted to say that you do not have to pay for fishing access. There are a number of Fish and Game access points on that river and you can go anywere you wish using them as an access as long as you stay below the normal high water mark. Not many people pay an access fees to fish the main rivers out here.
There are a couple of other rivers in that neck of the woods to try out. The Clarks Fork and the Blackfoot Both good fishing rivers to. Ron