the 6.0 PSD is about 80% International-navistar and 20% Ford. International designed the core of the engine ( all of the rotating parts) and sought a little influence from ford regarding bellhousing mounts, engine mounts, turbo choice, and electronic injection system
the old 7.3 PSD's were a little more Navistar and a little less Ford but still made by Navistar specificlly for Fords.
on both of these engines Navistar had made de-tuned models and marketed them purely as ther own.
here is Internationals web page for their regular 6.0 liter diesel:
VT 365
and this is the Powerstroke version of the same engine:
Powerstroke
Notice the differences in the output and oiling system. these are the same long block but have a different tune-up.
Also, Catapillar has nothing to do with International/Navistar they are competators. Navistar is simply the engine department for International, how ever you can order your International 18 wheeler with a Cat or Cummins engine or you can buy the engine from Navistar