how much horsepower can the 904 take?

The one I have from Performance Automatic is rated at 400 hp. It just has a shift kit and probably some better material but it's not really "built up".
Now, since I really need a book titled automatic transmissions for dummies, does that mean I cannot buy individual parts to make a performance kit for my '64 904? Also, I would have the work done by a shop, since I feel this type of work is beyond, my level of expertise. I need to know what to tell someone to do to it.