Tony - for what you need, I'd definitely go for the C4 before the C6. The weight and size difference is amazing. you probably wouldn't even fit the C6 in your trans tunnel without some hammering and cutting.....
The C4 will cope with all the power that you can throw at it.
You need a 164 tooth, 28oz flexplate
Fit the biggest trans cooler you can fit / afford. You can never go too big with these.
Without knowing your engine specs, I'd hazard a guess at a 10", 2000 - 2500rpm convertor. Much less and the car will lurch into gear, due to the light weight of the Cobras. Much more and it just won't feel right. Yeah, it'll probably accelerate a little better with a 3000rpm convertor, but you'll keep wondering if you're in 2nd or top gear when cruising through the suburbs, and it'll sound like a ski-boat when you get on it....
