Here is what I have learned through the years. It is better to have a slightly smaller carb than one that is too big. A smaller carb maintains higher velocity through the venturis at low and medium speeds, and thus the boosters get a better signal. The better the signal at the booster the better the carb works. As always everything is a compromise, the smaller venturis will create a flow restriction at higher engine speeds and thus they will limit how much power you can make. Everything else being equal, a square bore carb compared to a square bore carb or a spread bore compared to a spread bore, the bigger the carb the worse your fuel economy usually is, and the more drivability issues you will have. Although if you are running 4.30 gears, I doubt you care about fuel economy. When I built my last 340 duster in the 1990's I ran a 750 DP modified by Barry Grant to Flow 910 CFM, it ran great. That car had the 509 purple shaft, M1 single plane, ported and polished T/A heads with the T/A adjustable rockers, 10.5:1 compression, 4.30 gears, reverse manual valve body and a 3500 RPM converter. If I was running a similar setup except for 318 cubes, I would still run the same carb. Now, those old 750 CFM Barry Grant carbs may flow 910 CFM, but they would work a lot better than an 850 CFM Holley. They still used the smaller throttle blades and venturis of the 750, and not the bigger 850 throttle blades and venturis, they had much higher velocity through the venturis than and 850 CFM Holley. Those old BG modified Holley 750's ran on the street like any other 750 Holley, but flowed better than an 850 Holley, they were great carburetors, too bad they do not make them anymore. On a mild 318 I would run a 650, and on a street and strip one with 4.30 gears, 4 speed or high stall converter I would run a 750. Unless you are making a ton of power I doubt that you will see much if any difference in power on a 318 cube engine by switching from a 750 to an 850 carb, and the bigger you go the more difficult it is to get it to run right, especially if it will see any type of street duty. As for the boosters, annular booster can mask a lot of the issues of running a bigger carb on the street as they need a lot less booster signal to work, they will not cure a too big carb for the engine syndrome, but they will definitely make it more drivable and much easier to tune. Again, we come to the issue that everything is a compromise, annular booster are much bigger than a dog leg booster and thus restrict airflow, although they have the side benefit of atomizing fuel better than a downleg booster. If I was building your engine and I did not want to spend a fortune on a custom carb, I would just use the old Holley 0-4779. If I wanted to spend a lot of money, for honestly very little gain, I would would contact one of the custom carb companies that are out there and have them build me a carb specifically for what I had. My BG carb was built specifically by BG for my engine and it ran great, now having said that, I am sure that if I took a stock 0-4779 and tuned it right, it would probably run withing one tenth and a tenth and a half, and one mile an hour to a mile an hour and a half from what that BG did, and I spent easily 3 times what a Holley would cost on that BG carb 30 years ago...
