Another idea, the Lower Flathead. It is dam controlled, so should be running plenty long this fall. Put in at Buffalo Bridge (west of Polson), float to Dixon, or cut it short at another bridge (can't remember the name, maybe Sloan, 3 days or a fast 2). It's on the res, so you need to pay fees, but so few people do it you will not see many others. It is gorgeous and feels remote and is flat too. The fishing used to be poor, but supposedly it's gotten better since they eliminated dam tides.
Or you can put in at the dam and get Buffalo rapids also.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye