A lifter bore center can also be slightly off center to the lobe center line to cause lifter rotation with both the lobe and lifter faces being flat. I'm not certain which (the above convex method or flat) Buick was using back then. Perhaps someone here knows.
Measure. Are all the intake lobes identical? All the exhaust lobes identical? Measure all lifter faces. Flat? Convex? Same?
Welcome to every engine rebuilder's dilemma. What caused the isolated issue? Are the others (whatever, you pick. Valve, lobe, lifter, spring, bearing ...) ready to fail next or not and if not what caused the issue found?