This tip comes from Russ Carnes in California, USA.

It is possible to push the rear core plug out as follows:
    Drain the coolant.
    Remove or water proof the distributor.
    Remove the core plug over the distributor.
    Insert a long thin screwdriver between the edge of the core plug and rear cylinder jacket to hammer (gently) the rear core plug out.
    Clean the seat for both plugs well.
    Use the Dorman plug (or Betson Bolt for 5-main MGB) to insert the new rear plug.
    Replace the plug over the distributor.

Russ