Some ideas to your project;
Possibilities to use the space were the muffler sits. Reroute exhaust pipe through the diffuser or 180 degr out in the rear center. How do you prepare to evacuate hot air from the engine, louvers or nasa ducts?
I have also been thinking about the possibilities to fabricate and mount a flat bottom to the car (with a little angle) and a rear diffuser to create downforce. If you not are familiar to the book: Competition car Downforce, I highly recommend it. Also compare with:
Lotus F1 78/79
Have fun and good luck!
Yes I know this is'nt for a street driven car but I think it's a lot of fun and interesting things in downforce...