First, I would drain out ALL of the antifreeze, then put the head back on and VERY THOROUGHLY flush the cooling system with fresh water (you want to remove ALL traces of antifreeze). then fill the cooling system about half way with fresh water & then add 1 Quart of Sodium Silicate (aka: "Water Glass"), and then fill the cooling system up with fresh water. Start the engine and let it idle for about 1 hour while closely watching your temp. gauge (you don't want it to overheat). Check your coolant level & check to see if your leak has stopped, if the leak hasn't stopped keep idling the engine a while longer. - I have used Sodium Silicate to repair all kinds of cooling system leaks: cracked heads, radiator leaks, heater coil leaks, and cracked blocks. In FACT I sealed a cracked block that had a crack big enough that you could drop a kitchen match through the crack, it didn't seal right away but it DID seal.
IMPORTANT: Do NOT under ANY circumstances put Sodium Silicate into a cooling system that has antifreeze in it! The Sodium Silicate will turn the antifreeze into a thick gelatinous mass that will completely plug up your cooling system.