When you find a place to build you a tranny, ask them the difference in prices between a c4 and a c6. c6's are supposed to be the "beefed-up, high-performance" trannies, compared to the c4, but i've heard it can be better to have a c4 because you can make them tough enough to handle all the "excessive" power, if there is such a thing, and they said c4's are quite a bit lighter than c6's. I guess it just depends on if you are going to race your 66 or not. I have a '69 coupe with a 351 and c4, and it handles 375-400 horses just fine...
