Theres barely any room to work under the hood of a 6.4. Girlfriends father has one, and that thing is packed in there. I think the newer gen body style is bigger than our style? Sounds like too much of a PITA to me, unless of course you want to do this yourself, then you will have a payed for truck that sits for months at a time while you work out every little detail/problem that comes up. Just make your 7.3 a fun dd/tow vehicle. No need to make it a drag/pullin truck.
For the cost of the 6.4 swap, you would have one very nice 7.3L.