Running a four barrel carb on a two barrel intake with an adapter plate is a bad idea. The two barrel intake acts as a choke for the airflow from the four barrel carb. I don't recommend it.
My advice for building a 318 in a A100 for maximum torque and economy:
1st choice: Holley 4360 Economsater spread bore carb (can't be beat for fuel economy, primaries aren't very big but it is 450 CFM, find it on eBay) I run one on my 84 and it has dramatically improved my fuel economy.
2nd choice: Edelbrock Performer 500 CFM/Carter AFB 500 CFM carb (harder to find, but they are out there)
1st choice: Offy dual port or Edelbrock SP2P intake (these are strictly swap meet or eBay items, heven't been made in decades) Good for low end torque and fuel economy, bad for High RPM power. Have small intake runners designed to maximize vacuum for low RPM power.
2nd choice: Edelbrock performer. Good all around intake. But a buddy of mine who knows more about cars says that Weiand makes better intakes for Mopars. Never used one, can't say.
Pick the "RV" cam of your choice. When I built the 351W for my 84 Ford, my engine guy recommended a cam grind that is basically identical to that of the Edebrock performer package cam. Don't go nuts- look for a cam that makes power from idle to about 4000 RPM. Get real- does your van motor ever get above 4000 RPM? Where does your motor spend the most time? I will bet idle to 3000 RPM.
Windows- they're what make a van worth owning!