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Yeah. I would echo what @airhead said about the long stints on the Interstates. Having done a fair amount of 4-5 day road trips in cars on them I cannot imagine doing that much on a motorbike - even when the weather was good. If the weather is poor it will be a lot less fun.
I would recommend that you consider taking some cut offs just to break it up. Peel off the highway, take a more local road more or less parallel to the way you want to go. Lower speed limits but you will see more of the country that way. And stumble across some incredible stuff.
The immediate 100 yards either side of the Interstate is a bad representation of the US. We are more than just car dealerships, billboards and fast food joints.
You've done a few of these already, but give it some thought.
I have a colleague from Helena. Let me ask what he recommends for MT for you.
Speaking of epic, i'm in the midst of planning for a trip next year. Here's the current proposed route over 10-12 days (roughly 5 hours riding a day):
https://goo.gl/fY7Xbx