[Carl n Susan]

Not trying to discourage you, but the 6.4L was the first generation of Ford's DPF system. I found it significantly affected towing if a Regen occurred going up a mountain, which I do fairly often. It rarely seemed to do a "passive" Regen and consequently it would invoke an "Active" one. This consisted of adding fuel to both exhaust pipes (which come together into one) which affects fuel mileage and limiting boost to 20 whatevers during the cycle and that affected power going up a hill. The result was about a 1-2 MPG decrease compared to the 6.0L or 6.7L. The newer (2011 and up) 6.7L have a much improved DPF technology where "Passive" is the normal Regen when towing.

If you can swing it, try for a 6.7L if you can. The 6.4L is reliable just less fuel/power efficient. Of course if you are unconcerned about legality, you can do a DPF Delete and avoid the issue.