From Whitehall, the first 9 miles were MT 69/MT 2. After that, I turned onto MT 359 for 15 miles to US 287.
By the way, between Whitehall and Boulder, MT 69 is paved (but longer) than the more direct but 50% gravel road I took.

Seems like a good amount of water. I've had occasional gaps of 50 miles in Montana, but not much more. On the water filter question, I'd think it might depend a bit on where/how you do your overnights. Finding surface water is sometimes hit/miss, though if you plan things right, you can sometimes camp not too far from a river/stream. In that case, I've found a filter helpful since I can just fill/filter when camping. If you are staying overnights in towns, then I find a filter less helpful and typically try to carry enough water get between stopping points during the day.