When I did my windows and light covers, I masked the windows leaving a 1/2 inch border and sprayed the black using 3M trim paint. It stuck fine and hasn't lifted anywhere in the several years since I applied it.
Keep in mind that you should be painting the INSIDE surface of the plastic and so it will always be protected by being against the body work. You can also paint it with a brush this way and the strokes will not show up. Probably the easiest project one could do on a GT40.
Mark