You made a good choice. I think the way to go is take a 6.7 Cummins, delete it, put in a GM Allison transmission, and put it in a Ford body!
Hope your 6.7's are treating you well...It is a 6.4. The fact it has sludge means it's running so that's a positive. Sadly, owning many of these engines in my wrecker fleet I can tell you unequivocally we never had a sludge issue—the motors didn't run long enough to create any.