[Flathead Fever]

I think you already have the problem diagnosed but If you did have an adapter to pressurize the radiator you can take an ABS drainpipe rubber cap and clamp it onto the water pump inlet and seal the water pump. Then you can take a piece of hose, clamp a freeze plug into it that is drilled for a tire valve stem, clamp that to the head outlet, take your compressor and pressurize it a "little" and let it sit for half a day. Then crank it to see if it blows water out of a spark plug hole. Especially that clean one you have. Then torque the heads and see if that stops the leak. Definitely run coolant in your engine once you get this problem solved. Try not to take anything a part until you have identified the bad cylinder and run some tests on it. You could also remove the upper hose, take a compression tester hose screwed into the spark plug hole and pressurize the cylinder with a compressor and see if you see air bubbles coming out of the head outlet and then torque the head and see if that stops it. You might get away just tightening the heads but my guess is probabaly not because of electrolosys.

I am not a fan of copper gaskets because of the electrolysis from the dissimilar metals. Moden composite gaskets are much better. They are designed to expand and contract with the same rates as a cast-iron block and heads or cast-iron block with aluminum heads. This is what all modern engines use, and they can last 300,000 miles. The heads will stay looking like new inside if you also use coolant. Copper gaskets are for race cars with extremely high cylinder pressures. I would buy a new set of Felpro composite head gaskets. They come with a heat activated sealer. Clean the block and heads with acetone. Do not spray anything on the gaskets. Torque them down, run them through a few heating and cooling cycles and then torque then again.

The person that asked you about your elevation asked because the coolant boiling point goes down as your altitude goes up. For every 1000' you go up from sea level the boiling point drops 4 degrees. When it boils and turns to steam it expands to something like 1600 times its normal volume and that's when it breaks parts. You never want to let and engine get hot enough to turn the water to steam. For every pound pressure a radiator cap puts on the system it raises to boiling point 4 degrees, but an early Ford radiator can only handle 4 lbs. of pressure because of narrow solder joints on the tanks. Modern radiators can handle a lot more pressure and get away with running 210 degrees all of the time. The addition of a 50/50 mix of coolant also increases the boiling point even more, plus it prevents the electrolysis that eats your head gaskets and heads. It's a must that you run coolant, it's a cheap investment.