Some of the worst looking old cars in GT2 that look great in real life are the '69 Camaro and the GT350 and GT500. The designers really screwed up with them and did a horrible job, they don't look anything like they do in real life. The Camaro looks too bulky in the game, and the GT350 is too roof-heavy, and if you didn't know much about Shelbys, the GT500 doesn't even look like a Mustang, it looks like they took the '67 Cougar and made it bigger.
They also completely screwed up the newer camaros too, and just made them shaped like a doorstop with the aerodynamics of a brick.