Good eye! I think you're right. Looking now, they both have near identical vent window and windshield lines, I wouldn't be surprised if GM made them the same parts. But the Caddy has the mirror mounted further forward than the Chevy. Well done sir!

The mid-'60s was GM's era of brilliance, there were cars from different brands that looked very different, but, while not simply "badge engineering" like in the '80s, there was great similarity in architecture, with some different body or roof panels, and different engine. Astonishingly, the second generation Buick Riviera (rear wheel drive) and the first gen Oldmobile Toronado and Cadillac Eldorado (both front wheel drive) were built on the same chassis.