Headers seem to always be tight fits, even when they're in a car with a stock motor. When you put them on a modern engine in an older car, there are even more variables to consider than when putting them in a car with a period-correct engine.
I don't have any experience with this manufacturer, and I'm not saying that you're in the wrong here; I'm just giving my .
Of course, if the headers don't fit and Doug's is to blame, you deserve your back.
Good luck.