I think you will be fine with a pair of 350 cfm 2300s. Consider that the cylinder size of a 300 cube 6 vs 300 cube v8 is physically larger so there is more space to put more stuff so to speak. Also, don't forget that the 500 cfm is spread over the length of the manifold. The 500 figure is also at WOT. I think the slightly more important question with respect to the manifold is whether you have one of the earlier manifolds without a provision for water heat or one of the newer versions that does. The lack of heat to the bottom of the manifold will cause more problems than the carbs will with respect to tuning and drivability, in my very humble opinion.
