1971 Demon with some coolant issues

Well I was going to change the oil, refill the coolant and start it up, but.....

Drained the oil, left the pan plug out and started to refill the coolant. I heard a strange noise, looked back under the car and it appears that the coolant (I am just using water) is running straight out of the coolant system into the oil pan. I put a few gallons of water through the system and now clear water is coming out of the oil pan... This is not looking good.