Don't be afraid of the 6.4 Great motor once you un choke it. First thing you need to do is take all the EGR and DEF crap off it. Put a decent exhuast and chip it. It will pull like a 6.7 and get good fuel economy. Really, it will make a huge difference in how the motor runs.
I have a 6.7 F350 Single rear wheel short bed and I absolutely love it. I sold my 04.5 Dodge that I loved and this new Ford is way above my Dodge. I get 19.5 mpg and it tows our 12K horse trailer great. So much power and it rides great.