Craig,

The usual HeliCoil procedure ends up with the original size.  However,
anything is possible.

However, I suspect that your leak may be the drain plug gasket.  The
original gasket may not cover the HeliCoil.  Mine didn't.  I replaced it
with one made from soft aluminum as big as would fit.  Neoprene should work
great, but not last as long.  Rubber or paper would work as well, but would
have to be renewed each oil change.

Mike  '84 GL