Having broken a input shaft, I know they are a weak link in high HP trucks. When I called about one, and they were in stock everywhere, and were bringing in a Premium. $600-$800.
To replace one, you have to tear down the tranny until it's bits and pieces.
Last time I talked to ATS (about the Allison), They said the shaft was plenty strong.
That is one gargantuate shaft for the Allison!
Merrick