JD, the carb that will work best for you depends on what you are putting it on and how it is to be used. A 351c with a mild RV cam and top RPM of 6000 that is only driven on the street could use the same 600 CFM vacuum secondary carb that works on a 302 while if you are turning 7000 rpm and running on the drag strip as much as on the street then a mechanical secondary double pumper at 720 to 780 CFM might be called for in a light mid sized car. Your carb is just one piece of many that all work together to make your engine perform corectly under the conditions of use. To tell you any more than that without knowing a lot more about the car, engine, tranny, rear end, tires etc. would be a shot in the dark.
I have not had good experience with Holleys. I bought both a 600 and a 750 for my 351C CJ motor in my '73 Stang and had problems with both of them. I changed to an Edelbrock 1406 and have had no problems since then. This motor is bored 30 over with a mild cam. I then put a 1406 on my '69 351W in my Cougar htp. It did great. It is basically a stock rebuild. Both of theses are on 4 speed cars. I then put a 1406 on my '69 351W in my Cougar conv. It is an automatic. Likewise, great results. It is also a stock rebuild. I am sold. 3 different cars, 3 different carbs, same result! I also have a 4100 on another '69 Cougar conv. It is ok but, I am getting spitting and backfiring when I nail it. It is one I purchased from Pony Carbs and was not cheap. Not crazy about it at this point. Oh well, I guess 3 out of 4 isn't too bad. My personal experiences. Jim
