318 heads and combustion chamber size

You must not have read my post. I know not to miss match heads. Common sense. I have 920 heads with a crack. I need new heads but not sure if later headz that would have hardened seats would be open chamber my motor is 66 273. I don't want open chamber want to keep my compression close to were it is now with 920 heads. The heads my dad friend says he has have hardened seats so Im trying to figure out what heads would have those. Then secondly do all heads produced after the 920s have the large pushrod holes and air injection? I personaly don't care if they do or not. Not the point just curious about combustion chambers on 70s and 80s heads that have hardened seats..