Frank...401k runs a 650 SD. When his 392 was stock he dynoed at 376 RW with a real wide power band. That equates in the 445-450 range. The 650 will give you a crisper throttle response in the low (2500-4000)to mid range. It will lower the power band slightly providing more tq (20) in the mid range. The 750 would come on with about 15+ more HP at the very top but if you get out of it by 6000-6200 the 650 may provide the best street performance.
Guys use both and it is more of a personal choice. Nick has found more folks at the Pit like the 650 over the 750. The Holley HP is another very nice 650 for about $500. . I ran a 650 Race Demon on a 450HP 351W. Throttle response was "very snappy" , I mean instant, everywhere. The car ran like heck and never ran out of carb with a 6200 shift chip. Go to
www.prestage.com.
Under "Car Math" then "Engine building" There is a carb formula
there. Looks like a 392 needs 598 cfm at 6200 with 85% VE and 633 with 90%. The Demons actually wet flow a bit above 650.
Jim Marotta