I had a concave body line on my 57 Ford fender the same place, like this:
View attachment 1715083217
I never could get it "right" so I did my best and primed it. It turned out to be about a 5 footer, but that better than a 10.
I still hated those "last minute" headlights. They were supposed to be quads for 57 but the quad headlight law didn't change in time.