Tailshafts can cause a lot of damage when they break, have a look at this clip about 7:40 in.
https://www.youtube.com/watch?v=jAZDU7oibCk
Im not surprised Hardy Spicer have put rev limitations on their shafts, because when they fail it does a serious amount of damage.
My race car tailshaft is rated at 8000RPM and the bloke at the time at Hardy Spicer questioned my diff ratio and tried to tell me the diff ratio slows it down.
Really i said, so my engine is doing 7000 rpm with a 1:1 ratio gearbox final drive and the tailshaft is connected to the end of that so whats it spinning at ??
For a couple of seconds he was a deer in the headlights but the penny dropped and i got what i wanted ! It is a big tube i must admit but it is required.
Cameron the guy who owns the valiant had the floor ripped apart leaving a huge hole adjacent to his hips, the rear of the gearbox was ripped off and it broke the outside tabs off his engine block, lots of damage and lucky it didnt come through the floor.
Regards Greg