Michael,
A close friend in the Puget Sound area of Wasington State had his safe in the garage and with a wipe down and intermitent fondling he had no rust. The humidity there is quite a bit higher than Montana. With a golden rod or two and our normal fondling of our toys you should make it. If I was doing it I would just monitor it a little closer this year and see if a problem crops up.
Best,
Ron