Slant 6 Turbo 68Dart Project

I'm hazarding another guess. Seems unlikely you could leak that much exhaust without really seeing & smelling it under the hood. Might seem goofy but I'd put on a leather glove, maybe a rag, and try blocking the exhaust at idle while observing under the hood and feeling how much exhaust pressure it builds. Another idea is to make it steam by squirting water in the carb, or squirting some mixed gas in to make it smoke while watching under the hood. I know, more farmer type thinking.