Its not Area 51 its Big Mountain, Utah.
There is no Big Mountain in Utah, there is Big Mountain Pass.
Similarly there is Big Mountain Ridge in California as Big Mountain is a location in Alaska, Arizona, Montana, Oklahoma and Pennsylvania.
Of those places chances are its Montana as its North of Salt Lake and so would limit NCR from attacking the Legion from the North and Arizona as it close enough to Route 93, Route 95, I-10 and I-40 so the NCR cannot push into Flagstaff or Phoenix from the west.
Considering how to limit NCR movement I would say Big Empty is Big Mountain, Arizona as Death Valley limits NCR movement to west of Las Vegas but it could not prevent a NCR assault from the west into Legion territory (or Legion push into the west) as there is nothing blocking them, well outside desert but still you have Lake Hawusu and Alamo Lake to serve as stops, not to say the I-10 that goes right into Phoenix.
Great Divide might be a reference to the Great Divide Basin, also the Continental divide, now were its a issue ... I suppose Triple Divide Peak makes a good candidate, I have a few ideas about what might be there, does close up expansion of the NCR coming from the north.