Spoiler options
Have you considered a duck tail style spoiler? Probably won't be very functional but it wouldn't kill your lines.
I've considered it for my dart but haven't gotten to that part of my project yet, or if I would even like it. I always liked the one with the support bars in the back kind of the style of the drag racing spoilers.
Theyre buicks... I know, don't kill me but think they follow the same concept of originally "boring" looking cars with simple lines....
Top 2 photos are the style of spoiler that might work on an a body, bottom 2 are what I would be trying to get inspiration from