650 or 750 Edelbrock on 340
So a 750 @ 1.5 = 750, @ 1 = 612, @ .5 = 433
A 600 @ 1.5 = 600, @ 1 = 490, @ .5 = 346
Say your engine only displace 500 cfm of air, a 750 @ .65 = 494, 650 @ .85 = 489,
Both carbs are basically a 500 on your engine and obviously the larger is less restrictive but the question it that enough vacuum to get your carb to run the way you want to.