I like "the whole day reading and looking". Looks like a grand adventure.
For the Montana parts of the trip, if you have an extra day or two, once you leave what appears to be an off-highway campsite just west of Quake Lake, there are several routes worth consideration. The Centennial Valley and the Big Sheep Creek Backcountry Byway are each around 50-60 miles in length and are all graded gravel roads, with a single steeper and likely impassible when wet segment some 5 miles in length along the latter. At the north end of the Big Sheep Creek route, MT 324, Bannack Bench Road, and MT 278 and MT 43 will take you on a drive through the Big Hole, thence to MT 569 over the Divide to Anaconda, MT 1 to Philipsburg and on up to I-90 from P'burg. You may also branch west of P'burg on MT 348 to Rock Creek Road (FS 102) and follow Rock Creek for 41 miles to I-90 at Clinton. All graded gravel to the pavement at the 10 mile post, but this is a slow-speed route, so the entire 41 miles will take close to 3 hours.
Looking forward to following along!
Foy