One way I reduced the wind noise in my '73 camper but still
get lots of air was to cut circles into the fresh air plenum
under the dash.  I then fitted PVC threaded couplers into
the holes and sealed with silicone. Coupler plugs allow the
closure of the vents during cooler times. The fresh air  
increase is substantial and if done properly this mod hits  
where you need it the most!  Some purists may scoff at this
hack job but hell IT is your van and who else is going to 
want it when you are finished with it! 8{)

Bill
73 camper with its 1700 engine now making an awkward transition
to a 1800.