Follow Print Dad's advice and get a vacuum secondary carb, and for a stock engine don't go bigger than 500 cfm, if you are planning on performance upgrades in the future, then you could go up to 600 cfm. If you have an automatic transmission, then you need the manual kickdown linkage for the trans on the carb (holley makes an adapter kit for it on their carbs). If you are wanting an autolite 4100 carb like the originals, look at the numbers in the circle with the X on the side, a 1.08 is about a 470 cfm and a 1.12 is about a 600 cfm, although current air flow tests by independent shops have shown them to be about 40-60 cfm less than advertised, but they are still the carb requiring the least amount of maintenance. My carb on my 65 FB 289/271hp top loader 4sp is a 4100 autolite 1.12, and I've had it for about 40 years, and it has out performed all the other carbs I have tried, bigger and smaller cfm. Good Luck.