There are many great road and mtn bike rides in Montana. I live in SW Montana near Livingston and am more familiar with road riding. The Beartooth Highway is an amazing road and is usually open by mid June. It is a difficult climb, but in early summer with very little traffic. Send me an email if you want more info about cycling in the Bozeman/Livingston area: rebecca@lunatours.com
Good luck!
Rebecca