how much horsepower can the 904 take?

I am in research mode right now. I am trying to prevent starting the project and then find out that I cannot do this or that. Therefore, since I want to run a performance stall converter, and it cannot be done, I want to try something else, in order to make it happen. I am thinking that the pushbutton style is probably not suited for a performance set up. Do you know anyone who runs a 400 HP slant 6 with a pushbutton shifter? I suspect that they convert to a floor shift with maybe a B and M shifter. And, I am also not sure which 904 year would be a good candidate to convert to a floor shift style.