Might be far off here.. but if youve got her really shut down and its still runnning away either y'all have rocket fuel firewood or there is a air leak. Is it possible to seal up the exhaust with plastic bag n tape then blow low pressure/high volume air from a vac cleaner in the inlet. Go around all the doors n welds with a paintbrush n bubble mix of watered down detergent splashing it everywhere to see if'n it blows great big bubbles from somewhere. Hey presto y'all found yer extra air inlet hole, just gum it up with JB weld and see if the heat is more controlable without any extra air coming in.
What about a fat/dripping drain to the outside.. maybe its cocked open?