The 7.3, 6.0 and 6.5 were all sourced from International. The 7.3 was just installed straight up with little to no Ford-specific mods, and it was a great motor.
The 6.0 is where it all went wrong. In order to compete with the then-new 600lb/ft 5.9L Cummins engine, Ford took the 6.0, stripped most of the International engine controls and injectors off it, fitted their own equivalents, tuned the hell out of it, and kicked it out the door with similar power numbers.
Unfortunately, they didn’t do their homework and all the Ford-sourced bits ended up being pretty much a nightmare. Ford blamed International, who pointed the finger (properly) back at Ford. The same 6.0 engine, when used in box trucks and similar vehicles with the International controls and fuelling, didn’t have any problems. The 6.5 was a similar story, though not nearly as bad.
Didn’t this finger pointing take place about the same time the Ferd Exploders were turning over and Ferd were blaming it on their nearly 100 year supplier of tires, Firestone? What stellar ethics. As I recall the company was run by a brit a the time wasn’t it?
More specifically, Ford tried to make the HEUI injection system do something it wasn’t designed to do - split shot injection.
They were trying to compete with the quietness of the Chevy/Isuzu diesel.
Navistar told Ford that the HEUI injection system wasn’t meant for split-shot injection, but Ford thought they knew better and we got the horrible 6.0.
The 6.0L diesel fiasco cost both Ford and Navistar dearly - so much so that I’d suspect that when Ford announced that they were going to “do their own diesel,” there were many smiles and chuckles inside Navistar - something akin to “Well now, we’ll get to see how Ford does when they own the whole problem and can’t point fingers...”
For all of that, tho, I vastly prefer my 7.3L. It simply has better low-speed performance, especially with a manual transmission.