How I Done It:
First use a bodyman's contour finder to get the fender curve and brace location correct.....
Then transfer the curve to some cardboard template material.....
Then transfer the curve from the template to the slicer, note that if you follow the curve, the front and back of the alloy peice are out in the air. The vintage fenders had a much larger radius. So you can either raise the line up, and machine the alloy part thinner, or add to it so that it matches the curve. Since i didnt care about keeping the chrome intact, I added material to the front and back to match the fender.
Wish I had taken more pics of the fabrication, or maybe I want to keep a few secrets..... End Result: