Losing Coolant

[Kisner]

After I drive my 68 Mustang and turn the engine off, coolant runs out of the overflow tube for a few minutes. It's not a lot, maybe a half a cup or so. I'm puzzled as to what might be causing this. It has a mildly sooped up 302 in it with a mild cam and a 600 cfm Edelbrock carb and a 20-inch 3 row radiator. Am using a 13 lb. radiator cap and a 160 degree thermostat. I've been told to just add one of those aftermarket overflow containers but wanted to keep it as original as possible. Was also told to use a 16 lb. radiator cap but the folks at AutoZone told me it should have the 13 lb. cap and that running a 16 lb. cap would just make it run hotter. It's not overheating according to my temperature gauge, just curious what might be causing this and is it something I need to worry about.