My Father in Law had a '64 Rambler that sat for 20 years and had a (rust) seized piston. I can't remember for sure but I think I used Marvel Mystery oil in all the cylinders after removing the spark plugs. Let it sit for awhile then put a wrench on the front crank damper bolt and rotated the crank back and forth until it did a 360* rotation. He has restored the running gear and body and has been driving it for over 15 years since.

Of course you may have other problems such as a hydraulic lock and popping the clutch while pulling the car may have created a bent rod(s)

Remove the plugs and see if the crank will rotate with a wrench on the front damper. If coolant sprays out of the spark plug holes you have your answer about a "hydro-lock".

If the engine is now rotating freely,spin it over with the starter and listen for abnormal sounds. If you are confident that it sounds OK reinstall the plugs and drain the coolant and start the engine. You can safely run it for 15-20 seconds without coolant.

If it is passing this test go ahead and add coolant so it can warm up to operating temperature. Listen for abnormal sounds, if all is OK except for white smoke then go ahead with the HG replacement and be sure to check for a cracked head.

Good luck!