I haven't try to slip it in 5th but slipping in 3rd and 4th was enough and don't want to kill the clutch just yet.
I'll try to describe how it slips. I shift it into either 3rd or 4th, lift off the clutch which take the RPM to ~1400, and start to give it throttle at around 1/2 way down, by the time it's at ~1600-1700 RPM where the turbo lights then the RPM abruptly moves to ~2250-2500 RPM but the truck doesn't either accelerate accordingly or smoking tires, back off the throttle, it'll quickly drops to ~1800 RPM. Truck doesn't jerks, bucks or start moving sideway when the the clutch slips, it just felt almost like either I was free revving it or like driving an auto with the torque converter lock-up 'off'.
My truck have the following:
5x12 (90hp?) injectors
#100 fuel plate
16 degree timing
AFC modded for rack travel/tune-ability
4kgsk
Wastegate rusted shut
Mack plug (but it's irrevelant as I'm running a fuel plate)
My truck have 3.55 but Harrison's truck had 4.10 so I kinda think that's what helped the clutch from slipping. Although I wasn't even towing or anything and slip the clutch with no more than 1/2 of throttle.
If I remember correctly Harrison said the clutch only had ~10k miles on it before he part it out. Since it's still fairly new and good so I went with it. Although if it had 100k miles, I will scrap it before I'll even look at it.