The Summit carb is just a new version of an ugly Holley design that was so unpopular that they quit making it. It may be a good carb ( I don't know) but it's still ugly with a capital U. I would stick with a traditional style Holley and enjoy the benefits of good parts availability and proven performance potential. And 600 cfm is not too big for a mild 318. The cop 318s had Thermoquads and Quadrajets from the factory.
