Clayton, intalling an oversized plug is akin to jamming something where it
does not belong. It's likely that you have compounded your problem.
As far as teflon tape, it may or may not help, but teflon is used when the
threads are designed to provide the seal. In the case of this plug, it is
designed to slightly compress the copper seal between the two surfaces.
That is where  your seal will take place. You need to clean both surfaces
very well, re-install just beyond snug. You don't want to strip
anything......though it may be too late and, if so, you will have to
querry this forum with your new issue.
Good luck,
Miguel