With the 3.91 stroke in either the 400 block or the 440 block I would use the CPPA headers you have, I believe there 2.0 inch If not and they are 1 7/8 inch use them I have used both 1 7/8 and 2.0 inch in my Duster with the same exhaust sytem and saw no noticeable or measureable differences at the track or in the car