Heat and penetrating oils are good advice. If you can find a rice oil-based penetrant like MX-237 Master Oil its best, because its pretty much non-flammable and will continue to penetrate after the heat is applied.
What hasn't been mentioned yet is that in addition to moderately "shocking" the fastener by applying blows to the wrench (I use the palm of my hand, which is heavily callused, a mallet also works), you want to do so in BOTH directions of rotation.
Going in the direction that should tighten the fastener, but only in small bits, helps to break loose the locked threads. Even after you begin to see rotation, continue to work back and forth through a few degrees of rotation until you notice the threads really freeing up.
Be careful to increase the force used in small increments until results are achieved. Cast iron manifolds are relatively brittle in small cross-sections, such as threads. Too much force too suddenly can cause them to crumble.
Trust me on this

culero (gray-haired ASE certified Master techie)