You have the right number of carbs already. Dual carburetion is the wrong answer to a nonexistent question, especially on a 216. If you have the necessary engineering ability and access to a dynamometer, a TIG welder and a full machine shop, you might consider fabricating a 3-carb setup with very small single barrel carbs, similar to the setup on an Austin-Healey 3000 or an early Jaguar, using vacuum piston type side draft carbs for a motorcycle. I'm in the beginning stages of such a project for a 230 inline 6 cylinder.

BTW, I do have the dyno and the machining and welding capabilities to attempt such a project, plus 50+ years' experience building high performance engines. I'm not entirely sure I'm going to have the ability to do a good job of it.
Jerry

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