My opinion is that you are ok with what you have, unless you plan on some high (above 6000) rpm stuff. But given the stock cam and heads you won't be doing that or at least it won't have any performance advantage spinning it that high.
No I do not think so, if it was my car I would run a 600 on it. Your throttle response will be better with the 600 than say a 750. The 600 will receive a better signal than a carb that is to large for you motor. To much cfm will make your car sluggish in the lower rpm range because of this poor signal.