I expect it would be possible and allowed in MT. In my experience, these roads aren't inherently bad (wide shoulders, high speed traffic, not many exit/entrances), but I wouldn't do them for extended times if other alternatives exist.
However, that wasn't what I was suggesting. There is a ~11 mile section between Helena and Boulder where one either rides a (difficult) part of the GDMBR or one is on I-15. I did that part but otherwise was on smaller roads. I was coming from further north and then via Lincoln and MT 200 via Helena. One could also ride MT200 from Great Falls and then Helena.
Originally this started out as a San Antonio-Yellowstone run and then on to Great Falls to fly home ~ 2,000 miles total. By unlikely happenstance the wife of the couple in Great Falls was in Lubbock as I was passing through and requested my help on her drive home. So I find myself with some days lost doing the ride from the other end if time permits. My intended path is through Yellowstone to Cody WY. Ideally at Cody I have enough time left for the 1,400 miles back to San Antonio (20-25 days).
So at this point I mostly want to take the path of least resistance to Yellowstone.