Ahh, I'd assumed from a single view that it was slab sided all the way. 3-views clear up a lot of uncertainty.
One cheap way to find the natural streamlines is to carve a shape from soap or wax, and put ink dots on it that will be smeared by moving fluid. You can take it for a ride on your car hood, or put it in a flowing water stream.
As a first guess, I'd use the same shape you have from the center of the front wheels in side view, in plan view as well, to knock off the current corners. That should get you close enough. There will be some variation with crosswinds anyway, or there'd be no need for those radiused corners. It is fine for the air to fall off a "too sharp" edge at extremes, because you don't want that much side force to build up anyway. The curve gives a gentle stall, so there's no twitch, just better handling. Easy, na?
The side-marker/turn signal lights should solve any perceived apparent width problem with the headlights.
I recommend using Colin Chapman's "something for nothing" design, the venturi-bottom as on the Lotus 79
http://3lib.ukonline.co.uk/lotus/story.html You can start with a symmetrical wing design in Rhino, and squash the bottom against the ground so that the air goes across the top and bottom surfaces at the same speed. Therefore, there is no lift. The bottom still suffers from the wheels, which have to accommodated as well as possible.