Stock engine I would stay with the 500 CFM. Even then you may have to jet it down a bit. I have seen a lot of people over the years use 600 or 750 carbs for a little while then jet them down to 400. You will be able to tell when you hit the gas hard to take off and there is a hesitation then it goes like H---. This is a sign of to much gas at one time.
1968 Coupe <53000 miles on her 289 came off the line Jan 21st 1968. I bought it in 1983
2013 Shelby GT500 Born 12/06/2012 number 439 of 530 in DIB and 1 of 9 with Red Stripes over the top and on the seats