I have a 1998 c 3500 6.5. It has 259,000 miles. Engine had never ran hot. Blew out a freeze plug. Replaced freeze plug. Blew it again but a different hole. I flushed my cooling system with blue devil. Drove it 16 miles and it blew out another one. I’ve used brass, steel, and rubber freeze plugs. I have no water in the oil but was told it could be a d head gasket. Can anybody help?