I teach in SE Montana and live in N Wyo.
About interstates -
Bicycles on interstates are legal in most Western states outside of metro areas.
Each state has limitations published online on its DOT website.
Usually there is a wide shoulder - riding is generally safe.
But the shoulder often has rumble strips, road debris, and traffic is usually heavy.
US 212 is not a particularly good choice in SE Montana - western half worse.
Although it has shoulders for most of the way, traffic is heavy - truck traffic.
Truckers use US 212 to cut off extra miles from I-90.
There are service roads and Old Highway 87 from Crow Agency to Billings.
But the Billings area is not especially easy to bike thru.
If you are headed towards Glacier NP, then
US 212, MT 39, Old US 10 East, US 12, US 89 is the best route.
US 212 has lighter traffic east of Lame Deer, but some steep climbs.
MT 39 has moderate traffic but has been recently repaved north of Colstrip.
US 12 is a nice cross-state route (have done it).
Fairly light traffic except between Lavina and Harlowton.
The section west of Forsyth is pretty remote - bar/cafe at Ingomar.
US 89 over the Belt Mtns is a lovely, quiet ride.
MT 47 north to Vanada - Huh??
Narrow, dangerous - no paved connector to US 12.
Montana Bike Map:
Photo: Lower Yellowstone