Machine shop to have it planed flat and smooth. This can be done on a milling machine (expensive) or on a gigantic, glorified "belt sander" (less expensive, not as nice of a finish.)
That's IF the manifold isn't already cracked, and IF you use the manifold-spreaders before you loosen the bolts.
www.amazon.com/Lisle-13000-Manifold-Spreader-Chevrolet/dp/B0009OR94W/ref=sr_1_1?crid=1FR0UQ7CI4PIGsprefix=exhaust+manifold+spreader%2Caps%2C138&sr=8-1
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You'd need two of these.
They stay in place the entire time, from before the manifold-to-cylinder head bolts are loosened, until the bolts are torqued.
Without the spreaders, the manifold warps so that the ends twist toward the middle, and then you can't get the bolts through the manifold and lined-up with the holes in the head. This is so common that there was a time when GM cast a reinforcement bar into the manifold to prevent warpage in that direction...but they didn't do that on these vehicles.
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