This depends more on what your running behind the intake than on the intake manifold itself. What engine do you have and what's been done to it? If it's basically stock, the 650 might be too much, although not by a great deal. If I recall correctly, the stock SU carbs on the 240 & 260 ran about 225-250cfm each. 2 x 250=500cfm. At any rate, I'm not an expert on this topic so I'll stop here.
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