Loosen the fan belt so the pump can't turn--remove the thermostat housing and thermostat--pour the engine full as possible.
Time out for a word of prayer.These old engines were tough but prone to cracking both the head and the block.
Start her up and if all is well the water will be still but if it erupts like a fountain when you goose it--the head has to be removed.Then the truth will be known.
One word of caution--if the head must come off, there is a bolt hidden under the carburetor---take the carb off for access or the head will come off with great difficulty and in several peices.

Good luck

Bob Taylor