u/Mr_Emperor

So some historical context to guide your research for this scenario.

In 1598 the Spanish establishment the colony of New Mexico and began the Mission system to convert the Puebloans. The puebloans were already an agricultural peoples who dug irrigation systems to grow the "3 sisters" raised domesticated turkeys, wove cotton fabrics, made high quality pottery, and lived in substantial puddled adobe villages that functioned as fortresses when necessary (although they chose to flee onto mesas with as much food as possible if possible)

So the Spanish didn't need to introduce everything to the puebloans but the puebloans readily adopted, sheep herding, cattle, horses etc, wheat and vegetable crops, blacksmithing, carpentry, adobe brick masonry, water mills, wool weaving, etc.

By 1680, the domination of the Franciscans in particular and Spanish in general lead to the Pueblo revolt that killed 400 Spanish settlers, drove away the other 2000, and lead to Puebloan independence for 12 years. The spiritual leaders of the Revolt wanted everything Spanish, from Christianity to wheat, iron, and livestock destroyed. The Puebloans refused to give up the material benefits and eventually fell into factional infighting and warfare (the puebloans were never a single people but several peoples with different languages that lived similarly)

In 1692 The Spanish under Vargas returned, a handful of puebloan tribes allied with the Spanish and New Mexico was reestablished although warfare continued on and off for years.

In 1696, another revolt was planned for the same general uprising as 1680 but the plot was discovered, the ring leaders killed, and the most anti Spanish Puebloans chose exile to the Hopi, Navajo, Apache, and even as far as Kansas, bringing their livestock, weaving, and pottery with them.

So that's where we're changing history, **What if instead of another revolt, a mass exodus was planned, 90% of the Puebloan population (15000 migrants) gathered over the years enough foodstuffs, tools, animals, carts and in August 1696, immediately after the harvest, they begin moving northwards.**

Obviously the Spanish would attempt to stop them but let's say the Puebloans first hit the horse herds, stealing or killing the huge chunk and pursuit is hindered.

It's not a fast journey, they probably have to winter camp along the platte river near modern Denver before continuing but let's say 10,000 settlers make it to western Montana by 1698 with enough to reestablish their villages and farms in the mountain valleys of Montana, away from the Spanish.