I have a slight leak at where the speedometer cable connects to the transmission. I ordered some seals but I'm unable to remove the pinion to install them. I removed the metal tab that holds it in place and can twist it with a lot of effort with a pair of channel locks (without damaging the threads) but only in one direction and it does not move out. I looked at this video but it wasn't too helpful:

Can anyone offer a suggestion? Maybe thread something onto the threads and use a slide hammer or something to tap it out?

That O’ring on the external assembly has probably dried up and that’s your issue.

Slide hammer is not a bad idea, but good look finding that thread pitch.

I have vague memories of using a rag and a pair of vise-grips, with a hammer tapping sideways on the pliers. Or maybe a big screwdrive levering on it.

Anyway they do get stuck. Nothing special in there.

I found a new Z store.

TotalZParts

Datsun 240Z 260Z 280Z 280ZX 300ZX Speedometer Pinion Gear...

NOS (new old stock) speedometer pinion gear full assembly, including the sleeve, seals, and the retaining pin. It has a 17-teeth gear & measures about 3 1/16" in total length. 18, 19, & 20 teeth ge...

Try threading the cable back on and use a small block of wood and a wide blade screwdriver and carefully apply force on the coupling on each corner. Might just pop it out.

Thanks for the suggestions. I'm worried about damaging the speedometer cable by pulling on its. I'll give it a try.

I might try and find an old speedometer cable and improvise a pulling tool.

Your prying on the captive nut not the cable.

Edited Saturday at 10:57 PM4 days by Yarb
Adding

9 minutes ago, Yarb said:

Your prying on the captive nut not the cable.

You will probably bee surprised how it pops out. I’ve removed quite a few. There’s also a seal inside the cog shaft that prevents oil going into the cable. The ring and seal are still available.

Edited Saturday at 11:00 PM4 days by Yarb

There's also a keyway cut into the body. It was exposed once you took out the retaining bolt and the piece of metal that holds it in place. If you have a non-ZX transmission it sucks that it's on the top but you said you could turn it. It is a place that you could try prying...

Honestly, I've never seen one stuck that bad - and we've seen a lot of transmissions over the years - even series one monkey shifters.