727 part throttle down shift questions ?

I've got a 1964 727 (273v8) The only way to use kick down , is to go to full throttle. I know that at some point (year ???) mopar included the part throttle down shift in the 727's . What year was this done ?
Next question: Was the part throttle down shift changes all made into the valve body ONLY ? or was the case also different on the part throttle down shift system.
I would like to change over my 1964 727, to be able to have the part throttle down shift. But only if , all that I would need to do , would be to swap out the valve body.
Maybe someone can tell me if it can be done ?