I don't know about your specific application.  Spacers were used by OEMs for several reasons.  First, it allowed them to use the same intake manifold on a variety of engines, and then tailor the air cleaner mounting height with a spacer of a certain thickness, or no spacer at all so they could get the air cleaner exactly where it needed to be on specific applications (shaker, low clearance hoods, etc.). 

Second, it often just gave them a spot to hook up the PCV or other vacuum accessory of there wasn't a port in the right spot elsewhere on the manifold.

Third, it allowed them to put an aluminum spacer between what was typically a cast iron manifold and the carb.  Aluminum rejects heat a lot better than cast iron, so it reduces the tendency for fuel to boil in the bowls when the car sits on a hot day, which can help prevent vapor lock. 

The issue you are having is likely because the Edelbrock manifold is a universal part designed to fit the engine, not necessarily your specific car.  To improve flow aftermarket manufacturers often raise the carb pad height, which can mean you run into issues trying to use the factory parts from the carb pad on up (spacer, air cleaner, etc.).  As far as specifics, you might try these guys:

http://www.fordramair.com/new/info/faqs.htm

Performance wise, 6sally6 is right.  An open spacers effectively adds plenum volume to the manifold and makes it act more like a single plane intake.  It sacrifices a bit of low end for better top end.  A four hole does the opposite. When there isn't space for a spacer some guys will mill down the divider in the plenum of the intake to get the same effect.