I am just going on an assumed engine size and assumed max RPM of 5000 RPM. Assuming a stock motor, this carb CFM calculator (
http://www.carburetion.com/calc.asp ) says a 400 CI motor needs 491 CFM to turn 5000 RPM.
A daily driver van is not a rag car running 1/4 mile times. On the street, smaller is usually better in terms of cam profiles and carb sizing. Most of the time, a street driven engine rarely gets over 3500 RPM. For the best MPG and power it is best to go conservative on a street motor. Absolutely get the best flowing heads you can, but dual 2 1/2 inch exhaust is plenty, smaller cams are better than larger cams, and smaller carbs are better than larger ones (don't go nuts and put a 150 CFM one barrel carb on a 440, but a small four barrel is better than a large four barrel).
I used to run a 480 CFM carb on an offy dual port (yes, dual PORT, not plane) intake on the 351 in my old 84 Ford van. If I didn't have fun driving I could pull down 20-22 MPG on the highway, but I usually drove 75+ MPH and got mid teens. SIngle exhaust, I think it was 2 1/2.
But do your own research. There is lots of good info out there. Just don't believe the product marketing trying to sell high performance parts to guys drag racing cars. It is hype, and you are buying parts for a different application. You want low RPM torque, not high RPM horsepower.
Motortrend on Demad has a series called "engine masters" that has done some really useful videos about exhaust size, cam size, and carb size. Here is a great easy to understand instruction page from Holley:
http://documents.holley.com/techlibrary_selecting_a_carburetor.pdf