Using an open end wrench to bust loose a stuck nut is a good way to find the nearest emergency room. The open end will find it's way off of the nut when you least desire. I would recommend you invest in a 1/2" breaker bar and a good 1/2" socket set. A 6 point set is always the best, but for this tight app you might need the more versatile 12 point socket because it can attach in double the compass settings (I hope that makes sense, it's how I think of how a socket attaches).
For those times when the breaker bar isn't enough, a 1-1/2" schedule 20 or higher pipe will give you the edge.
I have found the breaker bar is almost always enough, and it's tough enough to smack with a sledge for a little impact action when needed.