running 273 at idle sudden large cloud of steam

Was running my 273 (66 vintage) and had a sudden very large cloud of steam, scared the heck out of me to be sure. The engine did seem to go rough idle just before it occurred. This engine has no hours on it and I just put it in last winter. Just installed a custom alum rad (closed loop), I added a puke tank. Have I got a blown head gasket? No coolant in the oil from the dipstick view. Don't want to start the engine again. When I look underneath there is a small amount of coolant drips on the K-member that was not there before. Not sure what else I need to add for help but I sure would like to know if something very bad has happened.:banghead: