gotta remember this car wasn't really designed to run sustained high speeds like that. Its body is formed to be slippery.. unfortunately slippery doesn't deliver much for traction or stability.
First thing I'd do is some sort of spoiler out back. That smooth roll off the decklid is a recipe for lifting at speed. You're not going to get downforce out of anything that doesn't look hideously like a race car piece but disrupting the airflow off the rear will help the rear stay put to a point. The nose needs some sort of splitter to keep air from gettin underneath and stay planted. If you work the rear, have to balance the front too. There are designs out there that are pretty functional as well as cosmetic (even more that are purely cosmetic though so be ware) .. At the speeds you can go legally you want to be safe as possible so functional pieces are more important than looks.
As much as I hate to say it too.. lowering the car will help to minimize under car airflow and as long as you dont go too low or too stiff. Gotta be careful there though as you dont want to make the car too stiff or bouncy.
Aero stuff is hard, especially because in most markets we cant go legally fast enough to need any of it. So almost everything you'll find off the shelf is just for looks
