I had initially thought that this transplant would be mostly a bolt-in - that was a stupid thought!!
Just got this back from PowerMaster: "It would be our suspicion that there might be something a little different about the aftermarket flywheel. Chrysler used the same starter for 20+ years on small block and big block engines all with the same throw distance."
Will do more checks tomorrow with the original 6.1 L flexplate to see if the ring gear is in the same axial position as the McLeod flywheel or not.
