stick with a 7.3. 6.4's have more problems than there worth. And removing the DPF to get better fuel mileage is a federal offense. 6.0's blow up every other day. if it weren't for emmision requirements on new trucks ford would still use the 7.3.
If you want real fuel mileage get a cummins 03 to 06 they kick @ss on fuel mileage.