I don't believe you can even get a sunroof/moonroof in the GTO. I drove a Sebring convert and it seemed to have plenty of headroom, I'm not near 6'3" though, 5'10". It didn't have any get up and go though. If you don't mind an older car, try out an 89 Lebaron GTC convertible with the 2.2 Turbo II. Has plenty of headroom, 5 speed, intercooled, and easy and relatively cheap to modify. Dash layout is quite dated though.