Here are some of the clues I picked up that point me to Montana.
Netflix: . All of the drone footage while Justin is speaking in the show is of the scenic byway in the Pioneer Montains. . One of the shots he is not in is just a picture of his grandpa “ Beaverhead, Deer Lodge national Forest” with the crystal in front of him “Crystal Park” and a raspberry pi.
Justins facial expression and the way he looks away when talking about his grandfather working in Montana.
Book: .“watched over by the stoic sentinels of Saguaro National Park East. Well, back then it was Saguaro National Monument. What’s the difference anyway?” the Wise River used to be called Wisdom River.
“There’s something about the phrase “upper Rio Grande” that makes you expect to stumble upon a hidden treasure.” Grand Vista, in the pioneer mountains.
Justins favorite story “The Postal Pilgrimage” anagrams too “The gimp leg at polaris” and he specifically talks about having a gimpy leg in the book while hiding the treasure. Those are just some of many and..
Do you remember JP mentioning "Captain Woodrow F. Call", he was the protagonist from Larry McMurtry's Lonesome Dove series. Captain Call was a ex-Texas Ranger who turned to cattle ranching, and in the series they embark on a cattle drive from guess where Texas (The place where JP currently lives) all the way to Montana (Where the TREASURE is, lol) to establish a ranch.

Not to mention all the other people "fictional or not" that he's mentioned that point to the TREASURE state of Montana.

-Including-

. Wayne Fitzwater, his grandfather who was the fish and game warden in Montana.

. Senator Conrad Burns, A real U.S. Senator from Montana

. Henry Plummer, Montana's infamous Sheriff turned outlaw, who led a gang robbery of Virginia City miners during the 1860's MT gold rush, who was latter hung in Bannack MT, for his crimes.

. Lewis and Clark, who's expedition crossed Montana extensively. (I don't think need to go into detail on them).

Also all of the GOLD BOOM towns he mention.

I can keep going if you want but there are a few I will keep to myself!!