Locked up Slant 6

As well, it may be locked up valve stems. So perhaps remove the rocker gear, and then you can check each valve to be sure the stems are free. If they are not all free then the head will have to come off.
Seriously tho;
The engine is, IMO, gonna have to come apart anyway with 40 years of moisture gunking up at least one and perhaps two cylinders.
If this car has a manual trans, make sure it's in neutral,lol