Cub Driver wrote:

It's I-90 through Missoula to Helena I think, then I-15N, but that's
a nit.

Thanks for the nit!

What are the high altitudes on this route, and how narrow are the
passes? If I ever bring a Top Cub home, this will be my route.

all the best -- Dan Ford

Do you have sectionals for your route? The interstates are clearly
marked (altho at times you may need a magnifying glass to find the
"I-90" lettering) as are many (but not all) the passes.

Another source of info are the specific states' Dept. Of Transportation
Aviation Divisions. Many states offer "not-for-navigation" aviation-type
maps identifying best routes.