How to remove water plug in head

[JcMercZ]

Has anyone got any ideas as all we have tried has so far failed.
Engine is old m110 280.
Basically just under the inlet manifold there is a bronze Plug i need removed as the head i had previously has a water return spout, this one is blanked.
We have tried the usual soak it with wd40 used a long bar, tried freeze spray, and a little heat (only on the bronze blank/plug). It has not budged!
Now im stuck!
Engine is in car, manifold is off. Plug is a flush fit in block, something like a core plug. It has a large hex/allen key fitting.

 

[GordonTarling]

Heating the plug will make it expand and tighten up. It may be worth trying a good dose of freezer spray on it. If all else fails, then drill it and use an easy-out.

 

[paris2463]

Plug removal

You are dealing with two metals which when screwed together will corrode "weld" if you dont require plug it is soft material drill out close to size last section then can be broken leaving thread in head intact hopes this helps