73 duster BB headers

44070dart, more than likely you'll need to move the engine up, down, side to side. This link should give a little more of what you're looking at. Assuming you are talking inside the frame headers read through the TTI info here. I believe the smaller tube headers will require a little less work or mods. Pay particular attention with the footnotes on the right.
http://ttiexhaust.com/Category-ClassicHeaders/400SR-218/TTi400SR-218.htm