Jon,

We take some new core plugs and use a -3 bulk head fitting through each one using copper washers as a seal, if you are thinking of doing it make sure you cut the backside off flush with the nut as the rear one is close to the cast of the combustion chamber...then press the new core plugs into the head....when we bleed the cars up we leave these off until water appears from them, to be sure there is sufficient flow through....the other two you can see are from the top of the rad and thermostat housing....

HTH

Ian