How much do you trust your engine? KGTF -D> KMMV is perfectly do-able VFR at 12,500. It'll look something like this:
If you don't like the looks of that terrain, you can go IFR - That is, the I Follow Roads variety. On about a 210 heading out of KGTF, follow I-15 either all the way down to I-90, or cut the corner from Helena to the west to pick up I-90. Go past Missoula, Mullan Pass, and just a hair past S83 Shoshone County you can punch in direct, which will shoot you about halfway between Mt. Adams and Mt. Hood. Absolutely beautiful country up there.
This will add about 70nm to your trip if you do cut the corner west from Helena.
Note: I do not fly in the area regularly (I'm from about 1000nm to the east!) but have flown there a bit (I did take that pic above). And I've driven I-90 through there many times as well. I can tell you what the terrain looks like, but not so much what the weather will do to you. Caveat Aviator.
Take lots of pictures of the museum AND your trip, and let us know how it goes! Both the museum and the flight will be spectacular.
The Hughes HK-1 (aka the Spruce Goose) is mind-bogglingly huge (That's a B-25 underneath its tail):
And be sure to spend a LOT of time at the B-17 with Lt. Col. Russell Barney, USAF (Ret.) who was a B-17 captain in WWII. You can't ask for better stories, or a better storyteller.
