I'm kinda partial to Caprice wagons, personally.
A Caprice/Roadmaster wagon, with the highest gears you can find to put in it (2.28 and 2.14 are easily swappable), aero mods, LRR tires, and engine tuning (software is free, all you need to buy is the cable) should get you easily over 30 MPG highway, approaching 35 MPG. And, do it pretty cheaply, easily, and reliably.
Add a manual trans and EOC, and you can consistently get 40+ MPG. Trust me, I know.
-Funkhoss