I have a customer with an 84 240 and the center two plugs are tight - no movement possible with a 3/8 ratchet. (he hasn't let me tune it up since he bought it four years ago, and the two plugs that did come out are Bosch, put in dry - serves him right) I told him flat that these last two will break off, in the same manner that yours has. He has half a tune-up as a result. To do it properly, the head should be removed, and the steel "bushing" cut longitudinally. With a little heat, it will possibly spring in on itself, and then you can wind it out. You can then install a helicoil - the tap will clear up any damage to the original threads when cutting the old one out. You may also find an automotive machine shop that will tackle the job with the head in place. The least of your worries are the bits that may fall into the cylinder - its the bushing removal that will pose the problem.