<avowed Ford guy... Hey, my two cents here. The 6.0 is fine if you dont beat on it or jack it up with a tuner only...As for the 6.4? My personal towpig is doing just fine, thank you, and drug a weeks worth of firewood, camping gear, and a 10k trailer with a 4000lb Jeep on it to Colorado ad back this past July ( stopped in Indiana for fuel-heat index 126!)....no issues, none. And not for nothing, but the Torqshift trans is as good as it gets.. Ask the diesel drag guys how long the Allison lasts when you turn up the boost. The Ford trannies have not had failure below 1100 lb-ft., stock. Dreaming of putting an Allison behind a PowerStroke is like trying to stitch pig legs on a racehorse...why bother?
GM sells their HD's on Allison trans (not designed or built by GM)...Allison is a vaunted name, but that particular tranny does have shortcomings in the light duty trucks, brought to you courtesy of the GM beancounters.
As for that darned DPF behind my 6.4? I still have it. Yes, Im aware the milage isnt what it could be, but then again I never thought about that much when buying a truck that I expected to tow a house and weighs nearly 7000 pounds. If mileage is your big concern, go rock out in a Prius, I'll be sure to give you a honk and a wave while i pass you going uphill, on my way to a camp and a trail, while you head to Bed Bath and Beyond..have fun.
..just my 2 cents.