I haven't had a problem yet at 104. 5K miles and YES I have the 53 block. Still I keep an eye on it though.
My transmission still shifts great too. I have the 6spd, though so that's not a direct comparison I know.
Just lettin' ya know that if you pick up an 01 that's been treated right, it may be just like mine. Doesn't mean things won't go wrong, but for now, i have no problem getting in my rig and hooking up.
Don't know about dually's, but my stock rig had right over 20mpg during the summer.