Read,
There are a couple ways you can go for whitetails in montana. The continental divide splits the state with the West side being mountainous and heavily timbered with lots of public land. I personally have hunted the Lincoln area south along the blackfoot river and up the drainages on either side of highway 200. The drawback is the timber, it is thick country, and can be quite steep. You don't see alot of deer in a day of hunting, although you can come across a very nice buck. The east side of the divide is more open country, and whitetails are found along the river drainages. This land is mostly private. This is the area i like to hunt whitetails in the most. They are found in the river bottoms, on out to the wheat field country up off the rivers with nary a tree in sight. It is more a spot and stalk game rather than stand hunting. Asking politely to hunt can sometimes get you on, or, The mt. dept of fish wildlife and parks also has a block management plan in place with private land owners. You can get a list of open block management land from the dept of fish wildlife and parks. They are listed by region, in my case, i look at region 4 and 6 and get booklets for both. They will give you a list of co-operative land owners, how to contact them, and a map of how to get there. They will also list what species the land owner allows hunting for, such as whitetails, pheasant, mule deer, elk, etc. You need to do some research, and mark out the ranches listing whitetails, and get a more detailed map of each ranch. http://fwp.mt.gov/default.html, this is the site you need to start with, you can also get info on how to get a deer tag.
Good Luck