If you wish to run any more boost than approx 15psi, then the best zetec based plugs out there will fall apart on you (risking serious engine damage). Obviously cossie plugs are available to suit any state of tune, so this is why the work needs to be done.

Its a very simple engineering procedure really, the head just needs to be milled vertically, shortening the threaded hole in the head so that when the cossie plug is fitted, it sits in the correct place. The spark plug threads are the same, so there is no tapping / re-tapping to be done.

Hope this helps