Highway 89 Montana

The "grade' would be Kings Hill summit.
I've driven then entire length and you should have zero issues.

There is some elevation to deal with in the Little Belt Mountains between Great Falls and White Sulphur Springs.

I found this link while looking for an elevation profile, should be good reading for your planned route.
http://lessbeatenpaths.com/montana-highway-89-south/

I drove 89 north of Browning last summer (see sig) and it's an interesting road. Not real wide, kind of curvy, and goes from 5000' to 5600'... not that steep. Not worth trying to bypass it.

No problem...nice drive.