Drill and tap the plug and use a bolt and a sleeve like a socket to pull it out of the block. Then either polish it a little, or fabricate another one that can be driven into the passageway without any danger of cracking the block. I have a 261 in storage that I'm planning on building into a "Cadillac 292" and I'm going to tap that hole and screw a bolt into it to block off the passage instead of relying on a drive-in plug.

The wrist pin lock bolts are 3/8-24, which should torque to 35 ft/lbs. or so. One precaution- - - -use something like a long tapered punch held in a vise to support the wrist pin- - - -if you clamp the rod into a vise and torque the bolt, you can twist the I beam section of the rod.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!