That is a "normal" problem with the old cables. New cables are hard to find and expensive. I have repaired old working but leaking cables by removing them from the trans, cleaning them well, and using the shrink tubing that has the sealer inside. On the shift cable, remove the adjuster, slide the tubing over the cable housing, and apply heat.
Yep. They also leak where the ferrule meets the sheath. Hose it off really good with carb spray, apply some rubber cement and slip a self sealing piece of shrink wrap over it and shrink it to seal the leak.
My 62 Belvedere sits quite a bit and that repair is still dry.
