Opinion on Clifford mods for Slant 6 motors

They tranny freshened look at spending $600 to $1000 (Depends on your area) The 904 is a stout tranny. A good converter another $600 or so.The rear is again what the price is in your location and gear ratio depends on the terrain you drive in your area.Local roads,hilly,or open highway.
All this also depends on how much of the work you will be doing yourself.