I would almost bet it's head gaskets also.

My advice, Get a 6.4 cam and valve springs while your in there and carefully check every lifter, roll it on a clean piece of glass and listen for anything thats not a normal rolling sound and replace the bad ones, even better just buy 16 new ones if you can swing it. Machine the heads to make sure they seal up real good, while they have em tell em to take .020 off, this will give you a smidgen more compression and a tiny bit more lifter pre-load to keep lifter tick at bay as the PRs are just barely long enough, almost not long enough. Throw in a new water pimp while you at it.