IIRC there is a coolant passage in the head that lines up pretty darned good with that second oil drain.. You'll need to plug that second oil drain in the block.. Tap it for a pipe plug and screw it in there flush with some sealer.. It would be best to have your oil pan removed so that you can flush all the cuttings from the tap outta there..
Next you'll need to contend with the coolant passage in the head.. Use the Hydro gasket with something cut out to fit in the hole for the extra oil drain to seal things up.. A piece of gasket or rubber about the same thickness would do.. A little hylomar sealer around the whole mess wouldn't hurt either..
*EDIT* You could probably use the mech gasket as well, but once again the little hole for the coolant would need plugged somehow...