I hit Run to the Sun last year. March in Myrtle Beach, SC. They claim +3500 pre 1990 cars…decent swap area. In 2022 I saw exactly one first gen Cougar… but a few gorgeous Nomads and two two well restored Marlins.
Carlisle PA Ford Nationals is the place to go. Several thousand cars and the other half of the grounds is filled with vendors. If you need original Ford parts with the right date codes etc, someone at Carlisle will have one!
Montana show you are thinking about is likely the International Mustang Meet which shifts back and forth between US and Canada in that region in Late Aug / Early Sept.
Then there is also the Sturgis Mustang Rally around Labor day weekend.
Mid America Ford and Shelby meet in Tulsa every June ending on fathers day,