running 273 at idle sudden large cloud of steam

forgot, think a couple of the exhaust manifold bolts go right into the water jacket - maybe ones just loose and the steam is from the water hitting the hot exhaust manifold

Well I was not clear where the steam was coming from was I LOL.
From inside the car it appeared to come from the passenger side front corner. No apparent leaks anywhere so I suspect that you may be right, a loose exhaust manifold bolt allowing the coolant to hit the hot exhaust maybe. It sure was a cloud and immediately stopped when I turned the engine off. Again no indication or leaks apparent. The rad is a brand new custom aluminum just put in, full of coolant. Oil dipstick shows no sign of contamination.
Should I start her up again and see where the coolant might be coming from? I'll for sure check the exhaust bolt tightness. Thanks so much for all the input and suggestions, I'm still a newbie but she's 95% ready for paint 3 years later.