Speedo Cable Connection

[magwakeenercew2jh]

I'm spending days chasing down fluid leaks. The trans (a 350) has been juicing all over the place ever since I made a lame rookie move and tried to fill it before I put in the driveshaft.
But, that's old news. The drive shaft's in. New seals in the transmission.

But more fluid keeps showing up. I *may* have it focused in after about a zillion trips under there.

The speedo cable has that knurled knob covering a flared tube which surrounds the "moving" cable itself. The knurled knob tightens down on a threaded fitting on the trans...
...or a fitting that bolts in to the trans that accepts the square "moving cable".

I don't seem to be able to get the knurled knob really tight. Threads are good. The knob screams for channel locks. But even with using them, the knob doesn't seem tight.

I was wondering if there is supposed to be a gasket or o-ring in there that would close the gap between the flared tube and the knurled knob.
I ask, because there seems to be some juice appearing at the gap.

Thanks.