Cut the intake surfaces of the heads, not the manifold. Then you'll be able to switch manifolds if the mood ever strikes you, and you won't have to have the new manifold cut (again...).
Would have worked well if I had picked it up before I assembled the engine (amateur!).
Too late now, and I don't plan on any changes - this will be the only SB build I do, the rest of my cars are Hemi 6 powered :)