Do we over carburetor cars?

I am wanting to buy a carburetor for a 68 318 engine in my 65 Valiant. it has a 4-barrel Holley carb and Edlebrock intake on it currently that came on the engine, but the carb won't work. I am wanting to replace the carb with like a 625cfm Carter carb, but one guy on here was telling me that a 525cfm carb came on 383 engines from the factory and would be plenty of carb. Is he right, and should I go for a smaller carb to get better gas milage? The engine has the heads milled .040 and a better than stock cam in it, don't know specs. It is just a street car.