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The "smaller" Rochester Q-Jets are @750 CFM - way more than a 305 would need up to say - oh @9000 RPM :D

Most Q-Jets used on SB's and other stock applications are the 'smaller' 1 3/32" Primary Venturi design 750 CFM main body units.
Lots of BB, HP and truck applications used the larger 1 7/32 Primary Venuri 800+ CFM main bodies.

If you are looking at different carbs at swap meets, etc. for parts or a replacement;
You can look down below the primary boost venturi (that 'double-roundy' loop area in the center) to see the main venturi restrictor circle molded into the wall - it will measure out ot one of the two diameters. If you can plainly make out that it protrudes out from the wall, it's probably a "small" (750 CFM) unit - if it is a small, barely noticable ring around the bore, then it is probably one of the higher flowing units.
 
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