a guy I work with, has a Vandura 2500 from the same era, same type windowless work van.. 350/ 700 R4 trans, and he gets about 7 MPG on average with it....

Buy a Plymouth Voyager work van or a Dodge Carevan Work van if you need a work van.. they actully made windowless work van's in the 1st and 2nd gen Mopar Mini vans which is IIRC 1984-1995.. with the 2.5L 4cyl they get mid 20's for MPG