The stock tranny is not as week as everyone seems to think ... I have literally pounded on mine for 125,000 miles (AutoCross, Road Race, Drags) and never broke it ... I was just starting to get some slipping on the 1-2 shift at WOT so I had it rebuilt... had all the good stuff put in it; 5 gear planetaries, Kevlar band, Raybestous clutches, Gorilla Sun Shield , etc. now I wouldn't say that putting Nitrous to it would be a health thing ,but; If you run a window switch (I'm using the MSD window switch) to shut the Nitrous off just before the shift; it should live a long life ...
the Impala Record is 10.02 @ 136 MPH on a 4L60E tranny ... He has switched to a TH400 now but if it can handle a car running 10's I think it can handle a little 100 shot of juice.
It's working for me .........
Kenny