I'am running a steel intake with three carbs and tube headers on my 250. You are right about the cold water and cold castiron. Start up is when you need the heat and the exhaust does a pretty quick job of heating right under the carb. Mine runs pretty rough for a minute or two till the heat starts radiating up to the intake. A few miles of driving and its good to go. We haven't had real cold weather yet but I suspect it will act about the same. The intake gets plent of heat with out adding more. My fuel milage has increased along with throttle responce. You may just need to rejet a little as adding headers will change the calibration of any engine. Check your plugs for real light or real black color (lean, rich). You can also fine tune by rasing or lowering the float a little. Joe
