Bitter taste of defeat! So much coolant in the oil pan.

can you give us a idea about the amount of coolant you lose in say a hour? A crack in the cast iron head or block would leak very slow! VERY slow! I would make a block off plate or three to block the water jacket, inlets on heads. So no water escapes when block is filled with coolant. then use a funnel to pour water into block with intake off. see if water still comes out drain plug. If not, then more than likely problem in the intake sealing against the heads. Or on further thought, just fill both sides separately.