Very belatedly, thanks for suggesting what looks like the solution to my problem.
A couple of questions....
1. Which is the best tool to make the initial selection of the cloak?
2. I've tried the Quick Selection Tool. If you think it's the best one, can I drag back a selection that pops out too far? In this case, out over the white background paper.
3. Any tips for making the background gradient feel more "natural", more like it was created at the time?