I deal with these on a daily basis, almost.
Cast iron is one of the worst items to extract pipe plugs from.
, get the right size 4 point socket (or 8 if not available). Pre-heat right around the plug
to about 400-700º as fast as possible then put a long breaker bar on it with a cheater bar and force it out. If the head
strips off, you never would have gotten it out anyway.
Now you'll have to drill and tap the thing out. and still not EZ, the threads will still want to remain. The method I found best is to drill close to the tap hole size, then take a carbide burr (rotary file) shaped to a sharp cone and grind in the hole until you start to see the tips of the thread. Then take a sharp pointed tool, like a scribe and start picking the threads out. Once you get most of them out you then take the right size pipe tap and clean the threads.
It's not an EZ job but if those plugs HAVE TO come out that's the only way it'll happen. Corrosion
has welded them in! I've done so many I lost
count years ago.
BTW-replace the plugs with a SS or bronze. The difference in metal keeps them from welding together.