The headstock tuner holes are indeed drilled wider to fit Schallers (I mislabeled them Grovers). So wide, my new Rick bushings almost fall through. At least the guy who butchered this bass left the old Rick tuner screw holes. They line up perfectly with the new Rick tuners.
I figured inserting dowels is the best way to go, although seeing as the holes themselves don't feel string tension (the tuner screws and shaft do) they could literally be filled with anything solid (15 min non-shrinking two-part wood putty), sanded flush, center drilled, and that could work too? Might be quicker/easier than dealing with dowels, and the work will be hidden once everything is back on.

