Ford designed the 6.7 and 6R140 (or whatever the last 3 numbers are now) in 2010, for the 2011 model year, for the F250-550 light/light-medium truck line. They also happen to put it into the F650/750 truck line, but I wouldn't say they designed it specifically for that line. It's what they had available in-house.
If they would have truly been designing an engine for the heavier trucks, they would have made a larger displacement, slower revving inline-6 (like the old Ford/Holland 7.8 and Cummins 8.3 they used in the F-, B-, C- and L-Series trucks).
