Yeah, that's my backyard. Where you from?
We did that stretch and circuit with Ride Idaho a couple years ago. Lovely ride although it can be wicked hot. It's a wide canyon, mostly exposed.
You might try to contact the route director at Ride Idaho (rideidaho.org) to see if they have any suggestions or comments for you.
Idaho's towns are strung together by two-lane highways that have been built over a seventy-year span. The maintenance is done by committed but terribly underfunded state departments. As cyclist, you will deal with debris, potholes and the simple fact that everything moves on the same roads you are pedalling. But if you've never been to Idaho, please take a few extra days to either layover on your route, go rafting or hiking, or stay a few days at either end of the trip. Idaho is a delightful place.
If you're staying in Boise coming or going, drop me a note. We'll get a beer or sushi or ribs or chinook.
Idaho is crosscut by fabulous mountains. These mountain ranges are separated by long valleys. Most of the valleys have beautiful rivers in them but the roads along the rivers are NOT flat. You can climb straight up and over the mountains or you can ride your bike around them along the rivers. While the mountains are spectacular from the river valleys, you only see the foothills from the highways. You must hike or mountain bike or horse pack a day or two to really see Idaho's mountains.
I run American and POW/MIA flags on my recumbent when I'm touring my home state of Iderho—pre-emptive political positioning.
You're going to enjoy your trip tremendously.