How are you turning the harmonic dampener? I trust you are using a breaker bar. I can turn my crank using a breaker bar with the plugs in place but it is difficult. Plugs removed and it doesn't take a whole lot of force. If you're not using a breaker bar, I don't think you'll get enough leverage with anything else.
Is there oil in the sump? If not, it probably is seized. If there is oil than it should be okay. Even if it overheated, you should have a cracked cylinder head gasket at the worst, not a seized engine.