is a blown head gasket the most likely cause of water in the oil in a 216?

A leaking head gasket would probably be the most likely cause of water in the oil. Another possible source is a cracked or broken rocker arm oil supply line where it passes through the cylinder block cooling passage from the left side to the right side of the engine.

Mark

Thank you Mark I pulled the head today and the gasket looks ok I have never delt with that supply line you mentioned so I will have to find a way to test it

Remove the side cover and you will see the line as it exits the side of the block and turns up and goes through a small hole in the head and on up to the center of the rocker arm.

Some guidelines: When working with a liquid that has entered another liquid, consider this. Compression is somewhere above 60 psi. The water pressure is basically 0 in a 216, and somewhere around 5 psi (-)(+) in a 235 after 1949. Oil pressure is somewhere above 10 psi. So what is my point? Water is not going to enter the combustion chamber unless the head is cracked as ~5 psi is not going to overcome 60 psi. Oil can go into the water if there is a void as ~10 pounds can overcome ~5 psi. I think you get the picture. So logic can help you diagnose a problem with mixing liquids.

I am not sure but I think there maybe a freeze (welch) plug behind the push rod cover plate. Another possible but not very likely cause is a cracked head under the rocker gear or behind the push rod cover. Another is corroded or cracked cylinder bore.
Tony