Keane - Got side tracked. Sure the C6 is o.k. to use. What I didn't say above was that the more power a tranny soaks up the more heat it produces. Unfortuatlly as a tranny ages this will get worse.
So How does all this stuff effect you? I would use the C6 bone stock(for reasons I will mention later) unless something is wrong with it. Use a ten inch converter that stalls to not exceed 2200 to 2500 rpms. I wouldn't use the stock converter for about 10,871 reasons. Gear the car so you stay above the stall going down the road in high gear. Make for darn sure that the radiator you are using is sized to cool your tranny. That obsolete front
oil cooler that most Cobras have and their owners insist on can be put to REAL good use, and be used as a tranny cooler. Your pipe path would be from the tranny to the radiator to the cooler then back to the tranny. Summit, Jegs and others sells tranny coolers that have fans for about 150$ U.S that can be used instead of a
oil cooler. Use Valvoline high mileage trans fluid or any other high quality stock type fluid. Don't use B&M's trick shift fluid. Red Lines
synthetic fluids are the only synthetics I have ever tried and I found the Red Line stuff didn't do anything any better than the stock type stuff, but they sure will take more of your money.
About 80% of most trannys problems and about 2/3 to 3/4 of a trannys performance is tied to the converter. You'd be wise not to skimp on this one because you will get what you pay for on this one. I swear on Turbo Action converters myself. Pay the extra bucks! You won't regret it latter.
I wouldn't put a lot of money in the tranny itself because I get a idea that later you might opt for a upgrade. That high dollar converter upgrade most likely can be used in a tranny upgrade later.
I wouldn't run a manual shift valve body but if you look around I bet someone would know how to drill the stock one so your tranny shifts harder.
If a manual tranny upgrade later makes more sense, in your case I think a short tail shaft toploader is a better chioce. It will favorably effect the resale, at least over here, BTW. And you will get back about 60 to 75 horspower to boot.
Remember you are about 100 horsepower down because of the gearbox, so weigh that very carefully when you tangle with that 450 horse Beamer.
Cobrashock