Last year in Montana we were in the Gravelly Range of mtns to the east of Dillon, and south/west of Ennis,MT. The roads in the area would work fine for your rig, and there was numerous places to boondock. Be cautious of the area though as it is frequented by wolves and grizzly bears, so a clean camp is a must. I believe you would be in the Beaverhead/Deerlodge National Forest. There is a office in Ennis and Dillon depending which way you come from. It is not too far from West Yellowstone. You can access alot of the area off the highway as you follow it from West Yellowstone towards Ennis. The Madison River is well known for fly fishing.