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Wow, something I have a tiny bit of experience with!! I modded a 1977 Dodge Van, 318, 4sp and eventually got a consistent ~30mpg.
As was stated by Aerohead the front dam can be flat or a bit taper back from the center with little difference.
Side skirts follow along the rear outer edge of the front tire to the outer edge of the front of the rear tire. Behind the rear tire I tapered mine in slightly, maybe 8 inches or so.
Two things that I did to mine that helped the MPGs:
1. I put two scoops on the belly and ducted the air out the back doors. The scoops were approx 3-4 inches deep and just in front of the rear axle, up thru holes in the floor and thru channels (originally dryer hose) to the exits in the back doors. When I refined this system I had a friend that did aviation sheetmetal actually build hard duct work.
2. I blocked off the grill with a sliding panel system that allowed me to close off almost the entire grill while in motion to let engine air come in from the lower grill opening behind the bumper. Or if in traffic open up the opening for free-er airflow. The engine driven fan was replaced by two electric fans for redundancy and the engine bay was vented thru a NACA type duct on each fender (which coincidentally would defrost the mirror in the winter!).
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