The TO-20 coolant level does not like to be completely full. It will usually blow out coolant until it settles to a level of about 1 inch above the radiator fins when you look down in. Trouble is, it is hard to see in there with
the hood on. That is how it is on my TO-20.
Not sure about the spark plugs, did you clean around them before replacement and did you torque them to any specification?