Hey, I am in the same boat you are... I just picked up a 460 big block and had to go tranny hunting to find something that can hold 600+ HP... I didn't want to go auto cuase it takes half the fun out of driving. Anyway, the trans I have settled with is a Toploader. They were in production from '64-'72 or somewhere around those years. They are designed for both drag, and circle racing. From what I have learned they are as strong, if not stronger then a c-4.
www.toploader4speed.com I think is the site I looked up to order from. A full out trans that will be strong enough for you should run you less then 2,000 that doesn't include bell-housing or clutch though. Check it out, it's worth looking into.
-Kevin