Al,
I run a c6 behind a 521. After 31K miles on and off the track I have developed a combo that works well. It has been a collection of "learning experiences."
As of now I use a nice big trans cooler ( about a square foot with its own fan ), a torque converter with a 1600 rpm stall speed built by FTI in Deland FL and a Carl Rossler built trans.
Both the converter and trans are important with respect to
oil flow for adequate cooling during 30 minute track sessions; either can restrict flow and cause overheating if their
oil passages are not large enough. The lower stall speed makes for less heat generation in the converter. For drag racing only, this is less of an issue and you might want a higher stall speed.
The c6 would be my choice for installing and forgetting. It doesn't weigh enough more than the c4 to make much difference and doesn't require any adapters etc for any big block.
The shifter is the 1965 Mustang floor shifter used in the original Cobras - it just looks at home.
Hope this helps,
Tom
edit: almost forgot - I use a 2.88 axle ratio. If your engine is a bit smaller, an AOD with its overdrive top gear is a better choice with about a 3.55 axle as the c6 has a 1:1 ratio in 3rd.