What tranny does it have?
fueleconomy.gov it looks like with the 305 (5L engine) depending on your tranny your EPA mileage will range from 12-14mpg in the city and 15-18 on the highway, so I would start out getting your vehicle to stock EPA, then improve from there.
I would start out by finding the narrowest tires you can get,and I suspect that narrow (light) rims for a van like that are going to end up being steel, just because alloy rims are tend to be wider, also if you get under your van you are going to notice the whole underside is not smooth to say the least, but I suspect that with the frame of the vehicle building an under tray should be easy and that is going to help your highway mileage.