Attach the crown molding to the ceiling only *not* to the cabinets.
Then, as the cabinets settle, you won't see the gap because they will
simply slide against the crown. If you attach them to the cabinets,
they'll pull away from the ceiling.

Will the cabinets really settle significantly? I'm going to screw them
to 80 year old studs.

How would I best attach the molding to a plaster-lathe ceiling?

How can I attach the crown to the unlevel ceiling and still have it
line up with the cabinets?