Mount Rushmore to Glacier

Mostly I-90 if you like interstates. We don't so for us it was US hwys and state roads. We also went through Yellowstone NP. This was back in 2010. You can go US 16 through Custer and then nw to pick up I-90 at Moorcroft. I-90 through Billings until you pick up US 287 and head towards Helena and pick up I-15, you will still be following US 287 actually, Take US 287 off I-15 at(near) Wolf Creek. Pick up US 89 at Choteau and follow it through Browning to the NP CG at St Mary or pick up US 2 in Browning and follow it west. There are other options.