If you're concerned with cold air getting back to the valves, add a short piece of exhaust pipe off the exh. manifold. A couple of feet should be plenty to minimize any pulsations that could draw cool air back to the exhaust valves. It's sort of an "old mechanic's tale" that running short or no exhaust pipes can warp valves. A leak between the manifold and the head would be more likely to cause problems than running with no exhaust pipe.
Jerry

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