quadrajet is Spread bore. The large secondaries "spread" from the little primaries, thus the intake has a funky little look to it.
Single plane intakes, are just that. There is one single plane that air/fuel enter and travel through. This means that all 4 barrels on the carb are feeding, all 8 cylinders of the engine. Sure this sounds like the best thing for everybody, but it is not. Single planes are meant for more upper RPM flow. They aren't great for low end grunt(though I Have seen some dynos where torque was actually gained with a single plane, that was an exception, not the rule). Single planes are usually taller and make for a straighter shot to the intake ports/valves.
Dual plane intakes have 2 planes for air/fuel to travel in. One half of the carb(one primary, and one secondary) feed 4 cylinders, and the other half feed the other four. Generally the left side barrels feed the right side of the engine and vice versa. These have more texture to the intake runners and help to atomize fuel better at low RPM. This makes for more torque on the bottom. Now Edelbrock Performer RPMs, have a slight notch in the plenum divider(the part that divides the intake into a dual plane). This allows for air/fuel to travel be fed from all 4 barrels to all 8 cylinders, but not a lot. This is one of the things that makes it a better mid range intake. Personally I just think it better rounded for all applications.
Now if this is a Port fuel injection system, throw this crap out the window. Port fuel injection(like the one previouslly pictured) or a TPI setup, should be single plane intakes all the time. There is no reason to try and atomize the intake charge when all it is is air and not fuel(we call this a "dry" intake). The atomization takes place, both out of the injector, in the intake port, and into the head chamber(like Vortec heads). Thats another topic but figured I'd note it.
