The small plug that's visible at the left side in Gdads' photo is for the oil gallery, and there's a corresponding plug at the front of the block. That's the passage that I like to thread for pipe plugs, as they are exposed to full oil pressure and I believe a threaded plug is more secure. That passage can be threaded with a 1/4" NPT tap without doing any drilling- - - - -just flush out any chips thoroughly afterwards.
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!