The stretch from the eastern border of ND through eastern Montana is the tricky part, and there is essentially only one route available.
Best bet is to plan fuel on the Missouri River at Bismark. I believe the fuel trucks will meet you there, I can get more specifics.
Next fuel stop is Fort Peck, MT, on the Fort Peck Reservoir. There's a marina there, and I also have a friend there who'll assist.
Then, Fort Benton, MT, just north of Great Falls is the next stop.
From there, you're going through the mountains, and there are lots of options from there on. Seeley Lake, MT, or Polson, MT on Flathead Lake, then over to Couer d Lane or Spokane on the river adjacent Felts Field.
It's a long ways across between some of those points. And, most of the Missouri River in that part of the world is now officially closed to seaplane operations EXCEPT for those specific places, so don't plan on landing just anywhere along there to refuel from cans.
Fort Benton can be a little tricky depending on river levels--it can be a little shallow, and the wind doth blow throughout that country.
It can be done, but SPA really dropped the ball on this one, by refusing to participate in the Missouri Breaks National Monument EIS process, which closed a couple hundred miles of the Missouri to seaplanes.
Need more information, send me a PM, and I'll give you names and phone numbers of a few folks along the way.
MTV