WELCOME TO THE FORUM! Nice to have you aboard. The senario you describe is not at all unusual.
The fans shut down with the engine. Is the radiator clear of excessive amount of bugs and debris so there's plenty of air flow thru the fins? Love bugs get horrendous and can stop up the radiator cooling fins. Is the coolant relatively clean and free of rust and crud? Do you have the correct amount of anti-freeze to water ratio, shouldn't exceed 50-50 mixture. If you don't live in Siberia USUALLY a 40% antifreeze 60% water will suffice. Also note anti-freeze enhances the cooling properties of water. Long shot here, but is the system clear of obstructions and is the inside of the radiator free from obstructions. Have you ever flushed the cooling system and reppaced the old anti-freeze with new? The thermostat could be sticking and if it has never been replaced do it and be sure to use ONLY the temprature Ford recommends for your engine model year. They're cheap and easy to replace on the Vulcan push rod engine and make sure to replace the gasket or O ring for the t'stat housing.
If the coolant level was low before you shut down then some of the coolant gets turned to steam.The reservior, (overflow tank), caps do wear out and should be replaced when a groove forms in the rubber. The system is designed to run with 16 lbs of pressure and if less then the water slowly dissipates. Also, you could have a blown head gasket which can cause air pockets in hot areas like the heads and/or block and when you shut down the bubbles fill with coolant and creates steam pushing the water out the reservior cap. Another clue is white smoke or steam coming from the tail pipe. First get a new cap and if you experience the same probelm go to the parts stores and ask for a blown head gasket test kit and check it out.
Fact: The number one cause of blown headgaskets is overheating so one way or the other get it fixed soon because the way it is every time you drive it increases potential for more damage. If it overheated in the past it only takes ONCE and you could very well have a blown gasket.
I believe there's a test kit where you take some of your coolant and put it in a container then add some of the test kit chemical and if the coolant turns a certain color that indicates exaust/combustion gases in the coolant. Let's hope not . . . Caps are cheap so do the new cap first and let us know what happens.
Suggest you take time to read this which better explains info I provided.
http://www.arrowheadradiator.com/pre...m_failures.htm