How many people recall the air dams that used to be for sale during the first energy crisis in the '70's?
Fiberglass with a rubber lip for the last few inches near the pavement. (see example)
http://i.pbase.com/g3/81/39681/2/89377443.k2jmfbT1.jpg

How about attaching a large lenght of thin sheet steel to the undercarrage of the van, so it has a smooth surface and an air dam at the front?
How about a roof spoiler at the rear to direct air behind the van?
Remember the old spoilers they used to put on the roof of stationwagons to keep the dirt off the rear windows?
Installing more aerodynamic side mirrors would help as well.
I agree with the idea that all the little bits add up.
Someone might want to check the Car and Driver or Motor trend magazines of the mid '70s. They did a story where they took a Vega and did as many mods as possible to increase fuel economy.
Todd
'88 Westy