Good info here on SW Montana: "In Missoula, the first choice for the wade fisherman is Rock Creek. Lots of public access with good fishing. You could spend two weeks here alone. Other waters would be the Bitterroot River, Clarks Fork and Blackfoot Rivers. While these are best fished from a boat, they all have decent access for the wader."
Vis a vis fishing in Montana their Fishing Reg book is like nothing I've ever seen on providing 'Who/What/Where/When solid information. You can get it on-line or the State may be willing to mail you one. One way or the other you'll want same.
One last thing: Your gas tank ...... don't let it get much below half full. It's a very long way between gas stations! I almost found that the hard way. Jeep tank holds 15 gallons and guy put in 14.9.5.
Or another way of putting that is I had roughly a quart left in the tank.
Well, one last thing. How long will it take to drive from Point A to Point B? Regardless of the posted (Interstate )speed limit, by time you factor in normal stops for fuel, food, etc., you'll only average 50 MPH. Or to put that another way a 800 mile trip will take close to 16 hours.