[Carnut427]

The big block chevy heads have two different port volumes. Half the ports are longer (therefore more volume) than the others. This is why they "stagger jet" the carbs in racing applications. By placing a short stack on a larger volume port and a long stack on the smaller volume port, the intake charge is equal for all cylinders, as the distance from the intake valve to the air cleaner opening, or in this case stack opening, affects the engine's operating range. Anyone here remember the "Sonoramic" Chrysler 413s with the ram intake with tubes so long they went over the valve covers, placing the carb outside of the heads? These actually kept the torque (lots of it) in the lower RPM range. Shorter rams are better for upper RPM usage. It actually gets more complicated than that, but this is enough to answer the question.
Any engine with equal-length intake ports, meaning most engines, need equal-length stacks.
Dan