Get a 24" 1/2 drive breaker bar from HF, a socket that fits the nut, an extension for the socket, and a 3-4" pipe that slides over the breaker bar, then soak the nut with penetrating oil and work the nut back and forth until it comes loose. Or heat the manifold around the nut with a torch until it is red hot, and use the breaker bar and socket to spin it out