The Viper box uses a 30 spline output shaft vs. the 27 spline output the F body T56 uses. The extra splines/diameter are so desperately needed on high power cars. Other differences are the Vipers uses cast iron shifter forks vs. aluminum in the F bodies. The gear sets are basically the same as far as alloys and heat treats go.
The T56 is nice in 12 second or slower cars, but when turning up the wick, its only a matter of time before something breaks. I have rebuilt quite a few busted ones around here. Some broke in 12 and 13 second cars. Most common failures I have seen are broken shift keys (since they are only stamped steel), broken output shafts (was from a low 11 second car), broken shift forks (2-3 usually) and other assorted broken cluster gears. The Vette boxes usually suffer from the shift key and fork failures since the shifter is mounted to the torque tube. It is very common for the Vette guys to break their T56.
Having a performance car with a T56 can get really expensive real fast. Keith Mease can tell you that one first hand as he has plenty of broken ones including one where the case let go.
