Best possible mixture distribution is always a high priority for Chevrolet (and everyone else), if for no other reason that it improves gas mileage. This is why minor tweaking was done, probably many times, as to the internal shape, carburetor position, plenum volume, runner lengths etc. before installing it on new cars.
The stock manifold received far more development than most aftermarket bits.
What an aftermarket manifold had to accomplish back in the day to sell:
1. looks cool
2. aluminum saves weight
3. possible power increase (slight loss to considerable gain, depending on other mods and tuner skills)
4. you get to wear the T-shirt, decal etc.
Remember that most of this stuff was designed prior to the SBC.. 1955.
