Something I used to notice on those thin valve covers (although you won't have the problem with the alloy cover) - it may also apply to the oil pan, given its age - Lay a straight edge on the mating surface at each bolt hole and see if the metal has gotten stretched upward over time due to over-tightening. If so, carefully peen each hole downward so that the surface surrounding the hole doesn't rise above the surface between each hole. Maybe even peen it just a hair BELOW that plane, knowing that when you snug-up the bolts the metal will pull up slightly.