Montana's a big state, so it depends, but unless you are way far north or south in Montana to start, I'd say take Mullan pass from Montana to Aurora.
In reality there are a lot of good route options in that area, but that pass is a non-issue in most any airplane (as long as the weather's okay) and is quite straight-forward. If it wasn't labelled as a pass, you probably wouldn't even recognize it from the air. Relatively benign, terrain-wise, and the highway is there. There's a weather reporting station located on the pass, as well.