Big Mountain is part of Whitefish Ski Area and in the summer you can take the chairlift to the top ($20) and hike, ride bikes and activate Big Mountain. The communication towers that sit atop the summit are easy to see from the top of the chairlift and it's only a short quarter mile walk up the graded road to the top.
This looked like a simple drive-to peak from the topo maps. However 1.25 miles from the summit I encountered a Forest Service gate blocking the road so I walked the remaining easy road to the summit. The views are excellent but the presence of large communication towers ruined any attempt I had at getting out a signal. I'm currently just using an 5 watt handheld on 146.52 FM voice. I've connected in this way up to 50 miles in the past. I had K7VK waiting to hear from me in Missoula and although I could hear him clearly, he could not hear me.
This is an easy drive-to summit on Forest Service roads. See link below for map of route. It will take you approximately one hour from downtown Plains, Montana to get to the summit. Four wheel drive not needed. The only downside are the large communication towers that will interfere with your transmitting. I found that if you drive to the far side of the loop road and get the towers out of sight, you can get a good signal to and from Missoula and up to Whitefish. Let me know when you plan to activate this easy one and I'll chase you as I live in Plains.