Kalispell has a Show and Swap Meet at the local fairgrounds mid-July. Runs 2-3 days. All makes and years. Fairly small - I wouldn't drive from SD to see it, but if you're in the neighborhood on vacation anyway............. classic cars, projects for sale, odds and ends.
Also in July, usually the week after the swap meet is the Glacier Rod Run, normally at the Dairy Queen. As the name indicates there is a cruise - usually up to Glacier Nat'l Park. All makes - some of the best streetrods in the northwest show up, some hotrods, some drivers. Attracts the goldchainers.
Just across the river in Mandan on June 13th is the Buggies and Blues car show. There's also other events that weekend around the town. I know sometime in August or September (I think) there are drag races up in Harvey.
The biggest car ahow around here is Burn the Point in late august, draws 15,000 classic cars, and 100,000+ visitors. Its an awesome show. Its two day event, and all the cars are classics. I think tickets run around $40-+, its a benefit for a local hospital.