I have 2 400s (one backup) for my Modified apsphalt Lefthander. 1 of them has the steam holes blocked off. My heads (Dart 220s) have the steam holes drilled.
Can I use these heads with my 400 that has the steam holes plugged?
And does anyone know why GM put the steam holes in the 400 engine if it works fine with them plugged?

Put the steam holes back in. They are there because the 400 block had siamesed bores unlike the 350 where water passes between the bores. If those holes are in the heads and on a 350 block they don't matter since the gasket/block aren't drilled.