Considering a '67 Dart GT Convertible as my first project. Am I insane?

Take a good look at the metal around the top trim with the snaps on it. Also check the drain channel inside the car where the top bolts to the back. If that's bad it usually takes some trunk floor with it. The drain dumps out in front of the rear wheels, that's a common bad spot. Just from the pic, I see a lot of rust repair in your future!