Yeah, I actually trust a 'bulletproofed' 6.0 (although i HATE the term..) almost as much as i trust a 7.3, but the thing with the 6.4 is there's nothing you can do to keep it from having more expensive failures than the same failures on a 6.0 (like a turbo!).
Noone's asking but i would consider 6.0 swapping my RV, and i already like the 6.8.
Ford improved the spark plug retention with the introduction of the PI heads in the 2000 model year, and then again in the 2003 model year. There hasn’t been an issue with plugs blowing out since these changes were implemented.
My 2003 6.8 RV blew out a plug while my buddy owned it previously. He bought the nicest set of plug inserts available (i think it was a $500-700 set at the time) and i talked him into letting me do ALL TEN. I had the intake manifold out and drilled out every single stock thread in all 10 holes while laid out between the front seats. It was murder on my back and stressful as hell, but it's FULLY, fully fixed. The stock 2003 holes had about 3 threads in them. I admit that that's fine as long as a plug never gets loose or overtorqued, but having no margin for error on a part that every monkey everywhere thinks they are qualified to swap out without even owning a torque wrench, is just a dumb idea. So we fixed it.
As far as all these Ford manifolds.. when Ford was first announcing the 'godzilla' 7.3 gasser i watched some interviews with the engineers that talked about how much work went into exhaust manifold durability and totally geeked out over it. I don't know how they're holding up now that they've been out for a while but the thought of how reliable and easy to repair the engine seemed to be.. was exciting to me as a mechanic. Dorky, i know.
Anyway, medium duty trucks are hell on exhaust manifolds. They operate at heavy load much more often than light duty trucks with the same engine (manifolds get hotter and heat cycle more often and severely), AND often have less or nothing to deflect water splashing up onto them than in a 3/4 or 1ton. Those heavy trucks going through a puddle can put a 20-30ft rooster tail up, and there's nothing stopping that same water from trying to take half the heat out of the exh manifold in about 1 second. Plus a v10 is a relatively long manifold to be a 1pc. If you look at inline 6s a lot of them have 2-pc exhaust manifolds, and the ones that are one piece have all sorts of little accomodations to movement in the design. My chrysler Slant 6 for example has a really persnickety set of hardware, and tightening sequence to hold the manifold no tighter than necessary to the head and allow for relative movement between the two surfaces. Plus it is a center-exit manifold which reduces the overall temp gradient from end to end. When you have a rear-exit manifold and you look at it with a temp gun it just gets hotter and hotter front to rear as each cylinder's exhaust is added to the flow, which makes the forces in the manifold worse than center exit.
The only time i ever straight-edged a used manifold, it was off a 5.7 Hemi. It had ripped out the corner fasteners, and it was warped SIXTY THOUSANDTHS! Like you could rock it back and forth on the table and go bang bang. If it had been one cylinder longer..sheesh.
