What year? What ignition system? What emissions equipment? What carburetion do you have now?
A dual-quad, or dual four barrel carburetor, manifold is in no way, shape, or form a fuel economy modification.
Simple things first, more difficult and expensive later:
(1) Inflate your tires to the proper pressure
(2) Do a full tune up including new cap, rotor, plugs, air filter
(3) Install a high energy ignition system. HEI is a good and cheap electronic ignition system that gives you a much hotter spark, especially if you use an E-core coil off of a late model fuel injected Ford motor. See HERE
or just follow this diagram:
(4) If you really want a different intake, I recommend an Edelbrock SP2P or an Offenhauser Dual-Port. These are 70s era technology designed to improve low-RPM torque and, therefore, fuel economy. Racers don't like these because the design of the runners restricts high RPM flow. However, how often does your van really get above 4000 RPM? I have a Ford van that has a 351W/C6 combo with 3.55 rear gears. I rarely get above 3000 RPM, even on the highway.
(5) If you are going to a four barrel carb, don't overdo it. I would recommend a spread bore carb such as the Holley 4360 Economaster carb. It is a carb designed to improve fuel economy, not performance. If that isn't to your liking, I recommend a 500 CFM Edelbrock performer carb.
(6) Headers- long tube headers work to improve efficiency and power. Just check if you can install then and still pass your smog inspection.
(7) Learn how to properly recurve your distributor for performance and economy. This is a somewhat complex but cheap way to get better economy and power.
(8) Synthetic oils in the engine and rear axle and synthetic transmission fluids have all been proven to improve gas mileage. At least run 5W-30 weight or lighter, unless you live in a climate that needs different viscosity.
(9) Stay at the sped limit. Driving over the speed limit decreases your fuel economy.
(10) Don't bother with premium gas. If your motor doesn't need it it is a waste of money.
(11) Make sure your timing chain isn't stretch
(12) Make sure the timing mark on your vibration dampener hasn't slipped
(13) install a vacuum gauge and drive to maximize engine vacuum under all conditions
(14) lighten the van- fiberglass parts, empty out unneeded seats and tools
(15) body mods to improve aerodynamics
(16) air tabs (only helps on the highway)
(17) Make sure the cold air intake is functional, if your van was built with one originally, or install one if it doesn't have one
(18) Make sure the heated air inlet system is present and functional
(19) Make sure the carburetor is adjusted properly and that the choke doesn't stay on too long
(20) Finally, you could bite the bullet like I did and switch to fuel injection. I swapped on a multi-port MAF-sensor factory fuel injection system off of a 93 Mustang GT. The power is amazing and the fuel economy is better too. This is by no means as simple as an intake swap, but it is out there if you have the resources.
If you are interested, I actually have an Offenhauser Dual-Port intake for sale that fits a 351 Windsor motor. Remember, 302 intakes won't fit a 351, and in the mid 70s Ford changed the number of intake manifold bolts from 16 to 12. My intake is for a 16 bolt motor, but I tapped and plugged the four extra bolt holes. Send me a private message if you are interested.