Knock them out and install marine grade brass plugs. Then you will never worry about it again. BTW these are not called "Freeze plugs" they do not pop out automatically if the block freezes. The block will likely crack if it freezes. Sometimes they pop out under pressure when that happens, but it's not a feature they were designed for.
the proper terminology is "core plug". These machined holes with metal plugs hammered in them are the final finishing process from the rough casting holes left behind from the supports of the casting sand that was used to create the water jackets when the block was sand cast. They are machined and plugged to hold coolant.
