This track did cross back into Montana briefly by Painted Rocks Lake, and could be modified to stay on the Idaho side if you wanted. It hits pavement around the lake, then gravels-out as you head west again. There is a isolated section of pavement, seemingly in the middle of nowhere that takes you up over the pass and Frank Church Wilderness
viewpoint. The turns to gravel again shortly after (west) of the pass. It is a nice twisty and entertaining section of forgotten pavement that sees little traffic.