Well, this probably isn't much help to you but a buddy of mine was way into these 500s several years ago and had tons of engine parts lying around. I know I've seen heads with and without the slots cut in them. It may have been a running production change Honda made to help oil the journals in the post '79 engines. If it was me putting this engine together, I certainly wouldn't be afraid of using the '79 head. Just keep the oil clean and full.