Need help

Ok I have this 1990 360 i wanna put in my 67 Dart but i'm not sure wat i need to do far as my headers. Right below the exhaust ports on the heads there are these lil holes and im not sure what they are. I don't wanna put the engine in the car and fine out the holes are causeing exhaust leaks. Can someone tell me what these holes are and do i need to close them up somehow and how do i close them if needed?