Does anyone happen to know the thread size on the gear shift lever for the gear shift knob?

I'm pretty sure it's M8x1.25. About 8 years ago I bought a new shift knob for my 260Z, and it wasn't the right size thread. I took out the M10 insert and swapped in one of these threaded inserts. https://www.amazon.com/gp/product/B002QUZNUO

It was a lot cheaper back then, too.

I'm not sure that's correct.  The one I ordered came with an 8mm brass insert and it's too small.  Could it vary by manufacture date or something?

You were right.  I got out my tap and die set and sure enough it's 8 x 1.25.  The top of the thread was damaged so I had to retap it.  Thanks.