Yes the convertible quarter panels are different. The belt line detail follows the top well on the convertible vs turning up and doubling back towards the front on a hardtop. The upper ridge also travels a different path, as I am sure you discovered on your own.
Have you noticed how wide the space is between the top ridge and deck lid on your convertible compared to a hardtop? There is a lot of real estate up there.
As far as I understand, the rear speaker comes with the tape player. The tape player also provides a fader feature when listening to the radio via the balance knob on the 8 track.
Looking forward to seeing your car when finished. Do you have the placement instructions for the rally stripe? I have it somewhere if you need that info...