Here, I can see that I have enlarged the passage: I can see some of the shiny edge around the flapper.

I have enlarged the main opening to be slightly larger than the outlet of the exhaust manifold: to not have a step to impede flow.

I have smoothed the side of the housing to round the portal.

I have more metal at the top to remove; and the housing is much thicker there, so I can remove more metal there.