i'd look at 30's race cars and perhaps planes for inspiration on the spoiler.
perhaps you could use this as inspiration for the back
i like how the whole back pannel is recessed and the trailing edges are all rather sharp, while still matching will with the smooth curves of the rest of the car
a gentle upswepth spoiler won't really damage the look of your car if it's blended with teh rest of the body contours.
and as far as retro style goes, a tastefull touch of chrome strips might, unlike the many modern vehicles they're applied to, look good in this car