Before you take your old cap off, take a ruler and a pencil, mark two intersecting lines to the screws of the old cap so that the pencil marks are also on the paint/metal of the tail. When fitting the new one, place on the top, put your ruler on the lines and where they intersect is your new screw hole!
For the whiskers: get a piece of wide masking tape and place it over the whisker holes. Cut/rip the tape so it ends at the bottom of the cap, and poke a hole in the tape where your current whisker hole is. Move said tape to new cap and mark.
You may also wish to add reinforcing to your beacon light with a light piece of sheet metal under(inside). I had to. The hardest part is putting those dang whiskers back in while standing on a ladder holding all the wiring.
