56 cable at the transmission end is different than the 57 . So the 57 will work but the 56 will not.
Lokar makes a universal, cut to fit, cable that works like a champ and looks original. The part # from Summit is LOK-SP1502U and it lists for $65.95. I just did the t-5 conversion in my 56 Merc and this cable did the trick. You will need to supply the proper speedo gear which can be purchased from any of the ford, Tbird or Mustang parts houses. There is also quite a selection on ebay. The speedo cable kit comes with all necessary hardware to attach the gear.
Pete