This is actually a great question. On a Dart I had, I went from X heads to Edelbrock RPMs and I found that the Doug's headers fit better with the X heads than the RPMs. Fitment was close, but not exactly the same. Then on my Duster with Hedman Race headers, I went from Edelbrock RPMs to Trick Flows and the RPMs actually fit a little better than with the Trick Flows. Again, fitment was close but not exactly the same. Sorry it doesn't answer your question.... lol