Lots of the aftermarket and factory High performance 180 degree manifolds used two eliptical slots instead of four holes. Most of them that I've seen had extra space at the ends, so that the radius of the elipse was well out side the edge of the secondary bore of the intended carb. I'm not sure they thought of the "pinching" action, but avoided it anyway. 

Frank/Rebop

Bristol, In ( by Elkhart) 

Frank's exactly right - the two big slotted openings on the Blue Thunder manifolds completely eliminate this throttle blade / bore issue. I've seen a modified ECZ-9425-B at a swap meet, that had the upraised Holley base pattern milled off flat (about 3/32's cut off) and the two enlarged slots milled down into the plenum. It didn't have the runner crossection advantage of the big new manifold - but definitely a major improvement over a tapered spacer - and would accept all sorts of 4 barrel carbs and plenum spacers. Should have bought it when I saw it - way cheaper than a Blue Thunder outfit.   

The Blue Thunder manifold also has a slot cut through the center wall between the two plenums at the approximate location of the rear throttle bores. This slot would tend to balance vacuum between the two chambers - for power brakes etc. 

 
 Steve Metzger       Tucson, Arizona