I live in Buffalo, Wyoming.
I agree that it makes far more sense to cross South Dakota and and Wyoming.
The Bighorn Mountains are stunning - esp in wildflower season -
but you will be too late for the knee-deep carpet of flowers.
Speaking of timing - it gets dicey in the Pacific NW mountains after Sept 15.
I'll start with SD - -
Highway 44 makes for an excellent crossing of the state.
No matter which route you choose, cycling through the Badlands presents a challenge.
If you want to stay on pavement all the way and not backtrack - you have to ride I-90 - bleah!
If you are willing to to and out-and-back - ride in the early morning from the campground.
If you are willing to do some dirt there are excellent longer and shorter routes.
Hwy 44 picks up traffic as you get close to Rapid City. Use caution.
Sherman Lake Road is a nice backroad from Rapid into the Black Hills.
I don't know if you want to see Mount Rushmore - I have never been there.
But you would have to loop back from Hill City.
The Mickelson Trail is not paved - but a fabulous ride to Lead.
In Wyoming, Hwy 34 becomes WY 24 to Devils Tower.
Then follow US 16 to Moorcroft - and the service road south of the interstate - WY 51.
Many people choose to ride the interstate west of Gillette - - try not to.
US 14/16 is lovely - - but start after the morning commute.
From Clearmont to Buffalo you have an ever-larger mountain range in view.
Crossing the Bighorns is tough - - you know you will have ridden over a mountain range.
I always suggest to people to go only 2/3 of the way to the summit from Buffalo -
Since there are a series of ups and downs - the seven steps.
Tensleep Canyon on the west side will knock your socks off coming down.
If you want to head directly to Yellowstone, you can proceed via Cody -
But if you are planning to hit the Tetons, you should swing south to Riverton.
Thermopolis has a state park with free swimming in the hot springs.
Then the Wind River Canyon is awesome as you head south.
Shoshone is a tiny town, but Yellowstone Drugs has the best shakes in Wyoming.
The road to Riverton is wide and somewhat busy - west of Riverton you connect with the TransAm.
That takes you over Togwatee Pass and into Grand Teton NP.
I would STRONGLY advise that you take a little south loop into the park -
Many rides do not - and it is a big mistake.
There's hiker/biker camping right at Jenny Lake.
Fabulous hiking - esp towards Leigh Lake -
Or you can hike up into the high country to Lake Solitude.
When you head north into Yellowstone, be aware of the big climb.
If you wait until late in the day, it will be in the shade and much easier.
I much prefer Lewis Lake over Grant - which is a huge zoo -
You can always camp at Lewis and hit the showers at Grant the next a.m.
I would like you to consider taking US 89 north in Montana to Glacier.
Given your time frame, you have plenty of time to do this.
I have ridden every road in western Montana - few are better than US 89 -
And nothing comes close to Glacier for scenery or riding.
Going to the Sun Road is legendary - you know.
Here's part of a diary that includes the stretch from the Tetons to Glacier.
http://www.crazyguyonabike.com/doc/p..._id=26428&v=O1
After that, you can follow the Northern Tier to Anacortes.
Please feel free to ask me about any specifics.
J