http://www.solusinc.com/pdf/2003-01-3377.pdf
Page 13 and 14 might be of some interest if you want an alternative to the boat tail.
The air dam is kind of a front lip going close to the ground. It will deflect the air to the sides so you don't have to smooth out the underside. It might be easier to do since you'll probably have a lot of area to cover down there.
Here's a nice air dam
http://forum.ecomodder.com/showpost....83&postcount=5