Here's the difference on the '63-6 vert W/S gasket

If you're replacing both, just push the windshield out with your feet. If you're trying to save either one or both, you can do it either way.....glass and gasket together, or, windshield first (probably the best way if the gasket is really glued in) and then the gasket. The convertible gasket IS different, but, the replacement one everyone sells will fit just fine. It's a slight appearance and coverage difference. Most people won't notice.