Hmm, that's interesting. The large injectors were only used on the early 4D55 motors, never on the 4D56. They went to the smaller style of injectors starting in 84 and used those up until the switch to common rail direct injection on the newer 4D56.
With the injectors they are completely rebuildable, we redid the ones on mine a few years ago. You can get new nozzles from Scheid Diesel, once you get those it's just a matter of getting the injectors shimmed to pop at the correct pressure