I have to agree with Otto. Just replace the plug with a proper one. Easy enough to get out by screwing a lag screw in and prying it out with a prybar. The replacement just taps in (after cleaning the hole.)
The JB Weld approach is a bad band-aid fix. Who's to say there isn't another hole ready to rust thru right next to the one that's leaking? Why do the job twice?
