The idle speed and idle mixture are the first things to adjust and are very easy. If there is an off-idle stumble or a bog, you'll need to make adjustments to the accelerator pump and possibly the power valve to suit your engine. Here is a link to a Holley manual that explains it in detail: http://www.holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf
from my experience the 750 cfm will dump too much fuel you should go with a 600 cfm. i prefer the holley. you should only go with a 650 or 750 cfm if you have 400 horsepower of more.
A 2-plane manifold such as the Edelbrock Performer or similar would be a good choice.
600 cfm will be enough for that engine.
Carburetor manufacturer's (especially Holley) are very good at helping their customers select the correct carb for a particular application. Depending on how the 305 is to be used, a 600 cfm is probably about right unless you intend to routinely rev it over 7000 rpm.
600 cfm no 1406.
how do you adjust the float level on a 650 holly single pumper
600 cfm should be just right for a stock or slightly modified 350.
The holley has more cfm`s cubic feet per minute than a regular carb.and it also puts double the fuel into the intake faster than a regular carb.
It really depends on the type of unit/make/model. But if you want a estimate, 400CFM = 1 Ton. So to answer your question, 600 CFM.
depends on the application, but anywhere from 600 cfm up to 850 cfm is common. Carburetor manufacturers provide a formula for this calculation.
Go to this website: www.innovatemotorsports.com , and go to their tech forums.
The difference is the 750 has larger venturis & throttle plates and flows 150 more CFM at wide open throttle than a 600. Depending on what models of Holley's you are comparing there could be other differences as well (float bowls, linkage, vacuum secondary vs double pumper, STD flange vs spread bore, etc.). Your question is very generic so it's hard to give an exact answer aside from the obvious size differences. If you had given the list numbers from both carburetors (located on the choke horn) I/We could give a more detailed answer of the specific differences between the two.