The one you show from Mike's won't fit your Special. The tanks through 1977 have smaller bolt spacing than the later ones.
Gotcha. Mine is a '79 F Standard.
So I looked further and found the non-vacuum petcock with the correct 46mm bolt spacing. None of the images show the actual flange however, it "Looks" right.
Interchangeable outlets too.
Am I correct in concluding that the only mod to the carb is to plug the vacuum port when installing these taps...?