John Kaase: "that's not a spacer, it's a base gasket".
The difference? He's important.
I think anything thick enough to matter will be detected.

Just based on laws of physics: we radius the bottom of the manifold's plunge into the plenum (I did very "directional" work inside my 383/343 HP 2 X 4 factory manifold based on the direction and proximity of the ports being supplied). I'm tempted to radius the carburetor exit below the base where it's not accurately placed on the manifold, or hanging above a machine cut. Taken many apart to feel a 1/16" ledge in places. In general, any surface interruption is a trip wire, and air stumbles and slows down as it goes past.
Care to share any info/pictures of your manifold interior??

Haven't inspected your carb, the main venturis are the intersection of 2 angles. Is their transition sharp or radiused? A small radius will improve flow, but try to avoid moving the "vena contracta" (smallest point) which is located about 1/32" above the booster discharge. Both ends can be enlarged at the same angle (steep on top, shallower below) making a smaller triangle with the same angles to increase venturi ID but it's Impossible to get in there. Easier to bore them out (yes, take the whole thing part) and make an insert. Much, much time for small gain.

Any re-shaping of the air horn, even intuitive blending the sharp edges, makes me very nervous: the factory calibration of air bleeds, blah is based on what happens there, and more CFM may upset the transition. Vizard has warned about this (but not specifically the Edelbrock IIRC).