I found it.
It's the Douzan Tunnel, and is located on Tahouzan in Niigata Prefecture.
Haven't you wanted to stay up all night dying to know that? I have...
Located at the foot of the mountain is Iwase Jinja (石瀬神社), which appears to have some association with Benkei that I am going to look up later. (I.e., when I'm not falling asleep and too tired to be digging through Japanese.)
So... how far have I watched into the movie? About two minutes. That's the point where I noticed that they give a shot of the name of the tunnel. If I had watched the movie instead of spending the time finding the tunnel, I would be done. But then, I suppose I wouldn't be me.