running 273 at idle sudden large cloud of steam

didn't put the wrong type of 4 bbl intake on it did you? I did that years ago on a 65 273, started up ok but when I took it for a drive it let loose, check your oil to see if it's milky. I got lucky and after putting original intake on I put in a bunch of STP, the
engine squealed for a while but straightened out - almost lost the motor.