In addition to the velocity-operated (AVS, Q-J, TQ) and counter-weighted (AFB) secondary air doors, some also have a dash-pot (vacuum diaphragm with a bleed hole) which slows the linkage activation as an additional tweak against bogging. These can easily be added to existing linkage and made adjustable by using a jet as the air bleed regulator. It can be open to the air (not filtered) and easily accessible because the air doesn't enter the intake tract.
IMHO some manifolds for staggered bore carburetors (Holley 4011, QJ, TQ) were simply made by altering the flange footprint and bolt pattern of an existing square bore manifold - but no thought as to moving the carburetor forward to improve the inherent front-to-rear bias. Has anyone seen attempts to re-locate with dyno or track results? This may even be legal (or at least nearly invisible)!