5.7 heads on a 6.4 short block?

No idea for sure, but sounds like a maybe:
5.7, 6.1 And 6.4 Hemi Cylinder Head Comparison - Knowledge Base

Sounds like the only reason the 5.7 and 6.4 heads wouldn't swap is the 6.4 has wider guide spacing and only works with a larger bore, but 5.7 should/could/maybe/might fit a 6.4 lower end? Wouldn't flow as well, obviously. Would also probably take a custom tune or stand alone EFI - which I'd do anyway ;)

The VVT is another matter, but doesn't seem to have as much to do with the heads as it does the cam drive and phaser.