"Edelbrock F4B manifold has the center piece between below pair of "carburetor barrels" lowered a bit.
I read that this will add 6 or 7 BHP…
When installing the carb, does the spacer also need to have the center section removed or does the spacer configuration make no difference?"

The manifold is split in half by the divider and is called a dual plane manifold. The reason for the two plane was to have each half feed half of the intake ports for the engine. This is so intake pulses from one cylinder do not interfere with intake pulses from the next firing cylinder which is on the opposite side of the engine. This is good for lower engine RPM idle and throttle response. However, at high RPM it was found that cutting a notch in the plenum divider increased HP in the upper RPM range by equalizing intake pulse pressure on both sides of the engine.

No, the spacer does not need a notch to match the divider.