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

On 11/2/2014, Larry Ingram wrote:
"It occurred to me that in the event of a rear core plug failure the old plug could extracted by removing the top center tunnel bolt giving a very good alignment to the plug. A long 1/8" drill or self taping screw could then access the plug for removal from the inside. It would be worth a try".

Larry