Well, I have spent alot of time around a couple of 800+ HP SRT8s that will have 1000 HP in the very near future and the shaft are holding up just fine. Now, if you have some ham footed moron dumping the clutch at 5000 rpm with super stickies on the rear...well there you go. There are 6.1 that have been bored to 426, have superchargers pushing more than 10psi, high lift/duration cams and reprogrammed but still running the stock bottom end. They haven't been dyno'd yet but 1000 is well within reach.