The Rochester QuadraJet was introduced in 1965 with the design engineering team at GM (which back then had about 1,500 degreed engineers working on research and design) having two totally divergent criteria to meet in one carburetor. First demand was for lots of power to justify the higher priced big block. The other was to reduce emissions (which has the side effect of improving fuel economy) as the federal government had already passed laws limiting idle output of pollutants.
So to meet these criteria the QuadraJet has two tiny primary bores that contains the accelerator pump circuit and the idle circuit which limits the amount of gas at idle. The rear bores are a variable venturi design; which will with enough vacuum beneath the car flow 780 cfm on top of a big block, or 640 cfm on a smaller 327 at wide open throttle. The limiting factor isn't the carb (it is basically the same one), but in the fact that the smaller motor can not draw enough vacuum depression to draw more air through the carburetor. As such it is the ideal carburetor, not too big and not too small.
I agree with Adam that the exterior of the Rochester QuadraJet is very complex appearing with all of the actuating rods and can be intimidating. The good news is that once the carb is set up; it stays that way for 100,000 miles. Problem is few are specialists in this carb. There are two national carb shops that do a good job in rebuilding them and configuring them for your size motor (these carbs were put on everything made by GM from a 262 V-8 to a 455 cubic inch displacement V-8).
For this carb to be tuned correctly it often requires custom made brass fuel supply rods, hand bored tapered brass main jets, and a pipe plug (with some epoxy) sealing the main fuel well. Once tuned it performs very well.