Wall-ceiling gap

My 1920's era house has a picture rail at the top of the wall. But the walls and ceiling are not absolutely straight (not even close!) so the moulding was put up with a sizable gap above it. This would not be a problem except that it causes a shadow in and above the gap which appears to be dirt (no matter how many times I check). Any idea what I can do to fix this problem? I was considering filling the gap with rope caulk.

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