I have Stanza struts and have a couple questions. Note im not worried about the spring perch.
1/ Is there any difference in the internal thread between 240k/other datsun inserts?

2/ What is the internal thread and pitch for the inserts, as to buy thread tap.

3/ Which model Datsun/Nissan/other insert do you recomend running. And approx length to cut down the Stanza struts.

I'll be looking at doing something like this soon as well. There was a thread a few days ago about this subject .
Z31 300ZX strut inserts seemed like a fairly good choice if you're looking for something firmer.
Would cutting a section out of the strut below the thread and welding it back together be a good alternative to having to cut a new thread in it?

Yeah, I cut below the thread and TIG welded back together - have a look in my photo album.
Cheers
PS - I used the 300Zx inserts as recommended by feral.