how much horsepower can the 904 take?

I know this is an older posting, with that being said, I am trying to decide whether to keep my 904 automatic or convert to an 883 manual. My 64 Dart Gt has the unique pushbutton variety. I went to the various vender links in this posting and noticed that the performance kits indicate 1966 and up. So, I have to ask, is there a performance kit for my 1964 904?