It is recommended to measure liner protrusion while you have the head off. Likely not such a big deal on a tractor without Jake brakes, but trucks will eventually start leaking past the fire ring, especially if liner protrusion is too low.
It may not be recommended, but it is possible to set the injectors without any Cummins tool. I've never used any special tools. An inch pound torque wrench is helpful, and I believe the procedure changes depending on the model. A good idea to back off the adjusters before installing the rockers, since the injectors are literally bottomed out at full stroke, and if anything goes back together different, it could damage an injector.
My 855 was leaking coolant badly around 1 head, new head gaskets was all it took to stop that, no machining required.
If yours has the plastic injector cooling nozzles(Right side top of crank case), evidently, they are prone to getting brittle with old age and breaking off, cheap maintenance to change them, can be done from the outside.
And the best part, the 3 individual heads are each light enough to handle by hand.