If it were me I would get the spoiler or valence and install the FG spoiler with the
cutout and the bumper. Now mark the bumper where the cuts need to start so
the cutouts will meet perfectly. Complete the tracing on the bumper ( I would
make a template from the spoilers cutout.) and find a metal shop with a plasma
cutter. Will come out clean and smooth. Maybe a little attention with some
sandpaper and have it polished and chromed.