One thing about the carbs mentioned here is they are vac. sec. carbs so,really its hard to over carb a engine with one, because the engine only opens the secondaries as much as it needs. if you ran say for example a 750 vac sec. carb on a 305, the engine more then likely will never be able to flow 750cfm, therefore the rear secondaries would never fully open because the engine will only pull them open as much as it can take.
Its going to run good with a 600cfm carb, snappy engine response because the primarys are smaller, and increase the speed in which the air enters the engine, it will work fine in a street car thats street driven.. no doubt. A 750 will also work just fine in street driving, its just it will provide more topend hp output,because it can flow more air if the engine requires it. There would be a small bit of low speed differance in the 2 carb sizes due to the differance in the primary butterfly sizes, but would be hardly noticeable and likely easily tuned out if wanted, you can change the metering rods and jetting to accomodate the engines set up , which needs to be done anyways as no 2 engines are the same and they dont require/need the same carb specs. I will leave the actual carb tuning out of this for now as it will just muddy up the water.
A mechanical sec. carb will show you a over carburation mistake quite clearly as youll just get served with a bog or the engine will lay down from too much air introduced too early from the mechanical linkage operating the rear secondaries,and not the engine load pulling the sec. open. You can tune out a bit of it with squirter size and such but never get rid of it and make it run like it should.
the mention of a quadrajet is not a bad one either, the reason they were used for so long by GM is they provided excellant engine response and lower speed fuel economy. The very small primarys providfed fast engine response and great low speed around town fuel economy from the small butterflys. The large vac.secondaries provided good topend power out put, it was kinda like running around with a small 2 barrel engine,but also having a great topend output.
so after that, it basicly boils down to what and how your going to use your zz4 engine, mild street cruising only,daily transportation, your likely better off to the smaller sized 600-650cfm carb and tune it in to the engines needs as far as jetting and such. If your a weekend warrior/hot rodder type with cruising the car, a occasional street race from light to light, trip to the strip for fun, whatever, you might be better off with a larger carb for the better peak hp output, it again will need tailored to your engines needs,jetting,metering rods if edelbrock,etc..
1972 hatchback, 28,000 orig miles, 427BBC/twin T4 turbo's/T56 six speed/big wheels, lowered down pro touring style-work in progress....
1973 vega wagon-under the blue flame knife.