The T-56 transmission is built by Tremec. It was used in several GM models, as well as a beefed up version being put in the dodge viper, ever since its creation. The viper is only offered in a 6 speed, and it is known to be quite bulletproof. On the other hand, Mustang Cobras, the "terminator" cars, came with a 6 speed as well. This is also a T56. However, it has small differences that render it useless to others, such as different input and main shafts. Different years, and models, have different characteristics, such is triple cone synchros, etc etc.
GM has since, in the C6 Vette I believe, progressed to the TR6060, which is an upgraded version of the t56.