I have shaved and smoothed several manifolds, 2 n42's. They are pretty simple to shave. You can grind down the linkage towers, almost all the bosses. I believe there are only a few that will actually go through the manifold. The 2 at the rear, one on the bottom, and one in the center between the runners. There might be a few more, I'd check.
As for the relief valve, Id consider a BOV so you dont need to drill a hole in the manifold.