Your new MC will have the part you now need. So problem will go away.
The threaded part of the "plug" has a standard thread. The part you are asking about is interchangeable for the the 240Z, 260, 280Z and up to the 1979 280ZX.
Parts wanted: What you want to buy is an old 240Z, 260Z or 280z MC which will give you two plugs and cones.