i would just check the water pump first, these can be really hard to detect when leaking, and only usually leak under pressure when running. the leaking coolant could then be blown over the engine top and into the plug well.
posted this link to show how the water pump leaks but the pics are gone,

. anyone got them saved somewhere.
http://igmaynard.co.uk/bongo/forum/view ... ak#p572266
you need to get a dentist mirror with an angled head and place the mirror under the pump shaft,then shine a torch into the mirror. you may see a clean slug trail where the leaking coolant has cleaned the casting. a pressure test on the cooling system does usually show a leak on the water pump as the pressure can make it leak, a vac usually seals it.
worth a try if you dont know how old the water pump is.
http://lushprojects.com/bongopartsmk2/c ... f02.html#2