Well, at least we dont have drivers with Alberta plates - -
Notorious all over Canada, eh?
I suggest US 89 & US 287 on the east side of the divide over Hwy 83.
Why? Less traffic, more wide-open views, and more sections with shoulders.
Here's the most recent Montana DOT Traffic Count map:http://www.mdt.mt.gov/publications/docs/datastats/trafcount/2009_traffic_flow_map.pdf
US 89 / US 287 has about a third of the traffic that Hwy 83 has.
Plus you have to negotiate some busy sections around Columbia Falls and Big Fork on Hwy 83.
I have ridden both routes a number of times.
US 89 / US 287 is far more pleasant for riding and scenery.
1500-1600 vehicles per day is not "light" traffic in my book.
It is enough traffic to require pretty good vigilance when there is no shoulder.