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I then opened up the manifold to match the adapter opening. I also radiused the edges of the manifold where it transitions into the runners. It is easy to rough in with the carbide burr, then followed up by a shoeshine motion with a thin strip of sandpaper to dress the edges.
Both modifications should aid in flow tremendously.
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I had done the same modification to my intake manifold that you had done for the carburetor flange. However I did flow testing with a head on a flow bench, intake manifold bolted to it, and a Holley carburetor on top of that. The result that I had found is the center cylinders did not flow as much as the end cylinders. However with the stock carburetor all of the cylinder flow about the same (and flow alot less than the Holley). The center intake runner make a sharp turn toward the cylinder. To resolved this, I removed the runner divider between 3-5 and 4-6. Now with the Holley all of the cylinders flow rate are close together.
The other reason I'm improving the flow was I modified the combustion chamber and intake port to improve the air flow and cylinder swirl. Before there was no swirl. Now with all of the modification I had improved the overall flow rate by 10 cfm at .450" lift and the swirl inside the cylinder is 900 rpm at .450" lift.