[elhigh]

Hot air from the rad has to get out somehow, and I don't think this model dumps that air through the cowl - in fact, I don't think any car does that? Dunno, but it would introduce a lot of hot air into the cabin. Pan the front half of the engine bay but leave a nice wide slot at the back to exhaust the hot air out.

The rear half of the car is a huge playing field full of opportunities for belly panning. I'm going to start doing my truck when the weather warms up, it's just a mess on the underside. NOTE: hi-po cars like the Mustang sometimes have such things as diff coolers and cooling ducts for the brakes, watch out for that on yours.

You could, just for fun, try removing the lip spoiler from the decklid. If your mileage goes down over a tank, put it back on. It looks to me like adding the kammback may not help you.

+1 on the wheel covers front and rear, and +1 on the wheel skirts. That's easy, low-hanging fruit. For attachment consider epoxying in some kind of little block that you don't mind drilling into. The epoxy should stick well enough without demanding you drill into your car's steel. I have the same misgivings myself, and my vehicle in question actually has rust-through in places - you'd think drilling wouldn't bother me, but it does.