According to my search here, there are numerous threads discussing the dreaded "in-block" coolant drain plug removal and subsequent replacement.
The plug is M14 x 1.5, metric pipe threaded and has a 3/8" square male head for removal . . . go figure!
Here's my recommendations for proper removal and suggested replacement of this plug:
First, go to Sears buy a Craftsman "3/8" 8-pt standard", 3/8" drive socket, PN 45792.
Second, go to any Auto Parts store and purchase a 1/4" MIP (NPT) brass drain cock from their "HELP!" parts.
Clean any gunk from around drain plug and warm up engine, then liberally spray PB Blaster around drain plug and let set until cool. When cool, use a "breaker bar", short extension and your new Sears socket to loosen that stubborn drain plug.
Put drain plug aside as a "back-up", you'll replace it after the block drains with your brass coolant drain petcock, which has the benefit of now draining your block with a mere "twist of the wrist!"