Came across this thread while searching for SP's big 12xxwhp dyno... Just wanted to add the type of clutch is huge. For example, a triple plate clutch although extremely expensive, will create far less shear force on the transmission. I'm not talking about holding power at all but how sudden it's applied.

 

Puck clutches = tranny killers

I killed one transmission in 1 month of my driving back in the day with a 4 puck (sprung), needless to say I didnt reuse the clutch, and no it wasnt in the Z but same applies when driven hard.