same as ryland said make a belly pan, i have a huge list of stuff that i posted on a thread about an rv earlier, it all pretty much carries over to vans
*grill block
*belly pan (you can go to rv parts stores and buy big sheets of fiberglass)
*tire pressure
*electric radiator fan
*the skirt thing under front bumper
*skirts around sides
*new shock absorbers/ springs
*make an angled roof extension (not to sharp of an angle) along the lines of a boat tail
*get rid of useless mass
*switch to lighter materials (heavy wood counters -> fiberglass counters carpet -> vinyl)
*remove radio antenna (relocate inside of body)
* DO NOT REMOVE MIRRORS!!!!! they are extremely valuable, just make aero housings for those
i cut some of the things from my list i posted for the rv as i dont think they apply to your situation
here is the link to that post if your interested
http://forum.ecomodder.com/showthread.php?t=1051
also in that thread (i think its the 3rd or 4th post) there is a pic of a van done by nasa, it has some really good ideas.
when it comes to efficiency of the motor, remember it is one big air pump, remove as many restrictions from the intake and exhaust as possible (switch to a high flow cat, ditching it really is not all to beneficial) also do all the regular maintenance (spark plugs, fuel filter, air filter, 5w20 oil)
anyhow hope this helps some
John