I don't have the "best" route, but for what it is worth:
- In travels between Helena and Old Faithful, I avoided US 287 between Helena/Three Forks because of the following report (and one other): https://www.crazyguyonabike.com/doc/...page_id=467156
- Instead, I came via GDMBR alternative route along short stretch of I-15 to Boulder, small road (small bits unpaved) across to Whitehall and then across via 287 and Ennis/West Yellowstone. These other stretches were all reasonable roads and Boulder/Whitehall was a nice out of the way ride.