I've done the full conversion before. You will need to cut out the mounting plates from the A-pillar to be able to mount up the kneebar. For the dash, you will need to remove the middle support brackets for the dash. They mount to the unibody under the windshield. Hard to describe, but you can drill out the spot welds and weld it into your USA Rabbit pretty easily. The side, screw mounts from your USA rabbit will work with the holes on the Cabriolet dash.
Some welding involved, and the knee bar will make your car heavier, but also stronger in the case of side impacts. That bar in there is extremely beefy.
If you could find a 1980-1984 Jetta lower dash, you'd be in better shape in terms of welding. Because the Jetta is unibody, it doesn't need the kneebar. Instead, it has a lower dash with an extra glovebox. Kinda hard to find these days.