I have got to thinking and trust me this always cost me money. I have stated a number of time if things go south, go the the last thing you fixed before the dip. will in the case of this 72 Seville it was the water pump. the car had sat for sometime with what must have been straight water in the block. the pump that was on it had had rust about half way up. so much that it bend the impeller when we started it. the only thing separation the water pump and the timing chain is a steel plate. it was pitted but look solid. I'm going out to were the car is tomorrow and pull that water pump. my bet is the plate has a hole on it now. as water is pouring into crank case when you fill the radiator. just can't see that happening in any other location. it should look like the last picture.