Jeff,
I did the same thing on my '74 rear fender. After cutting and de-burring, I cleaned the underside of the fender and painted it flat black. Before doing this, I taped the top of the fender; about 1/16" back from the cut, so the paint would "wrap" the cut. You cannot see it, unless you put your face there, and because it was taped off it looks intentional anyway.
Good luck,
Fabricator