[68RT]

Don't believe there is an o-ring. If I remember correctly, fluid is able to go up the cable. You could clean and plastic coat the bottom to stop the leak.

 

[AHBGuru]

Speedometer pinion has a larger o-ring:

Mopar Dodge 6035709 Speedometer Pinion Housing O Ring 1966-74 A B E-Body (at https://www.yearone.com/Product/chrysler-b-body/fd15752 )