You not only need bigger heads with the 347, you want a good intake (better than your Cobra) and more cam than you think you need.
Since you are asking these questions, you might ought to consult a good shop that's done this 100 times already. They can steer you in a better direction. It'd be a shame for you to fork out all the money it will take to do a good 347 and then have it make like 350 HP.
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