Your first picture shows the freeze plug pretty clearly, even if it's out of focus pretty badly. I'd suggest changing all of them while you have the manifolds off. Use brass plugs if you can find some the right size- - - -they don't rust out.
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!