Baumann engineering says a 4r70w should hold up with a few upgrades. And it's available with the right bolt pattern. They's changed there website and I don't see the high horsepower stuff listed, but it would be worth calling them.
http://www.becontrols.com/tech/ch3aodeupgrade.htm
http://www.becontrols.com/tech/ch3aodeupgrade.htm