I'm curious how you will be compensating for the lack of a roof.
Is the convertible body the same as the coupe below the roofline, and the roof just adds extra (but unneccessary) strength? Or are you going to base it on a convertible and weld on the rear half of a coupe roof? Or are you going to strengthen a coupe floor to make it as strong as a convertible floor?
A jigsaw to part 6 - job done
Scratch that - I just read that you are starting fom a convertible. Personally, that would be the way that I would go. It would seem harder to do, but it avoids the problem of having to deal with part 7, and different doors.