Sometimes planing doesn't fix it permanently, cause your removing metal which which makes the planed manifold run that much hotter. If your going to keep it, I would do headers. It's so much work to have to do it again someday. The manifold cools mostly through the head via the coolant flow. If you give your radiator and engine a good flush that can help. But your idea of extra air is a good one too. You sound like an old timer like me :) Remember the days way back when we didn't have warping manifolds or brake rotors? They cheapened up the metal by removing the nickel content, and it been nothing but trouble ever since. Good luck with the job. My knuckles ache just thinking about it. Don