The easiest would be the A&A Part Throttle Kickdown kit, around $100.00-I think. That was what I did on my 67 Dart. Well worth the effort! The car is much more enjoyable to drive with the part throttle kickdown.
That would be easiest. I did some looking around, and A&A only lists this kit for the 727 trans. I've emailed them to ask if it will work on the 904, but I'm not optimistic.
