That has to be the weep hole. Maybe some racing organizations require a pipe plug to be installed in there. If it's going to leak, it will leak as soon as you fill the engine with water. The engine won't have to be running. Fill the engine with WATER for the first startup. If you do have a problem, water is a lot easier to clean up then antifreeze. Once you confirm that everything is sealed, then you can drain the water and fill with coolant.