Chuck, the 64 had a 300 in it from the factory, and a 3 core downflow radiator. The shroud was gray fiberglass (Fremont car). In 65, the Skylark with 300 switched to crossflow radiator, like the GS has, with a plastic shroud. The depth from front to back is what makes the GS shroud unique. I have tried to trim 65 300 shrouds and it didn't work, or look, very good. Someone did reproduce a 67 GS shroud a while back, which would likely work, but is almost certainly different than the unique 65 GS version. Not sure if 66 is same as 65, but it probably is. Good luck in your search. BTW, I'm not new here, but rarely post :-).