If that shroud was larger, it would be more effective. I had a similar though smaller gap around mine on my '63, and found a seal for it. Go to restorationspecialties.com and look up part # BZ634. It's a push-on weatherstrip that will attach to your shroud and has a 1.240 "lip" on it. Figure 8-10 feet of it to surround your shroud.
On plastic shrouds, I clean them and armorall them.