There is a very small lip seal around the speedometer drive gear shaft that should prevent leakage even if the cable isn't connected. They're bad about failing, however.

Another source of leakage that's often overlooked is a tiny weep hole in the center of the sliding yoke at the front of the driveshaft. It's there to keep air pressure from building up in the yoke, but if the fluid level gets above the centerline of the output shaft due to the torque converter leaking down, fluid can seep down the splines and find its way out the yoke vent hole. The simple fix is to braze up the hole in the yoke to keep the fluid from leaking out in case the converter leaks down.
Jerry

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