It sounds like the crank snout is damaged. A repair sleeve is normally installed onto a harmonic balancer, but not the crankshaft snout.
Remove the harmonic balancer. Feel and look at the crank snout. If it is worn at all or the key is loose at all you need another crank shaft. Or at a minimum you need the snout welded and the keyway repaired.
This is a very difficult repair that can warp the snout alignment with the centerline of the main bearing journals.
I would recommend tig welding the snout to buildup the diameter, fixing the keyway, and having a crankshaft machinist regrind the snout to the original diameter.
Better yet. Find another crankshaft. It will keep loosening up. Remember, there were many years Chevrolet didn't even use harmonic balancer snout bolts.
And I agree with the previous post, replace the harmonic balancer with another one. No wear is permissible. Remember it's a non-moving part. It shouldn't be worn and if it is that is because it has a problem.
