Hey I am aware these cars have great potential for higher fuel efficiency regardless of their size, It looks like maybe you're the first here who will be heavily modifying one of these whales and finishing it. I have a Wagon too but it isn't as aerodynamic as the 1991-1996 Whale type. Someone on here a while back was going to convert one like your own to Diesel but didn't finish, I'm sure you can get 30+ HWY with a Diesel and Manual Transmission. You got a good start, I'd like to see more on subject of Wagons!
P.S. I once got 28.56 on a HWY trip in a 2000 Caravan. That's a lot lighter than the Wagon you have but the EPA does rate it at 19 Combined, whereas the Wagon is at 18:
http://ecomodder.com/forum/showthrea...tly-29880.html