I think the 6.4 is a VVT engine,now I have not kept up with those the past 8-12 months but last I knew people had not figured out how to put a VVT motor in a non VVT car,that being said maybe its not a big deal going in an old rod.

doing a 6.1 is easy,one call to hotwire and they will build a plug and play harness to make it work.