Yo SnowVmax4,
I use a C6 behind my 521. It works fine
My mantra is "
if it matters what gear you're in, the engine's too small"
The current trans (#4, long stories) was built by Carl Rossler, Girard, OH and seems fine.
I use it on the sports car track as well as the street, so it gets quite a workout. Wouldn't swap it for a manual trans!