I cut 2 1/2 inch holes in the fender wells and made a fiberglas duct box that runs from the duct opening to the leading edge of the well. Post this picture I placed mesh over this opening to keep the critters out. There are flapper vents inside to regulate air flow into the cabin. Works like a charm.
You can fab a flapper door from sheet metal pretty easily and use foam as a seal. To open and close it, you can use a vacuum motor and a vacuum solenoid to control it. They are availabe from VintageAir's catalog. Click 32.......Replacement System Parts. Here is a pic of the parts:
calc the volume of your opening Bill, you should be able to get by with a 2 or 2-1/2 hose. You may loose velocity with 3", not to mention space savings.