64 Dart 273 with 4-Speed Speedo Cable Leak

I don't remember there being a seal in there. Just and "O" ring around the metal housing end where it goes into the tailshaft. If oil is leaking out of the speedo cable housing, you may need a new cable housing. I've seen some that were resealed with some good results. It's a matter of getting the housing totally clean. Once clean, use some of the liquid tool handle dip to reseal the housing. Just make sure to let it dry and cure completely before it gets subjected to oil again. If the housing is severely damaged, you'll probably need to get a replacement.