Intake and exhaust will give you a few extra MPG in a best case scenario. The secret to improved MPG is EFFICIENCY. A low restriction intake and low restriction exhaust TYPICALLY improve efficiency. So do synthetic lubricants and such. A taller rear end gear will lower your RPM at speed, but your engine may actually working HARDER to maintain that speed, hence using more fuel.

To be fair to your van, start with a complete tune-up by a reputable shop. Something as simple as a fouled plug can contribute to crappy mileage more than you may expect.

Besides that, get rid of the big box and go for a little rice-burner econo car. If you're like me you'd rather have an American V8 than a front driver ricer, even if it does mean less MPG.