I've done this on several Raleigh/Hercules 3 speeds, the BB shell is 71mm, my buddy turned one down to 68mm for me with the proper park facing tools. For another I had a Ti cartridge BB that didn't have flanges on the threaded portions of the BB so I could just screw them in far enough to clamp the Cartridge BB and they are a bit inset, I did also use a JIS MTB spindle on my ex wife's conversion, and did not face that BB shell to get it to 68mm, I have no idea what spindle it is though :-/ BUT with every Raleigh I have converted to a square taper crank I have always had to have the cog as far outboard as possible to get good chain line, so I am confident when I say that you want the shortest overall length spindle you can get.
As for the threads, the BB shell is 26TPI and anything square taper is 24 TPI, I have some old steel cups that I cut notches into the threads of and I screw them in and out of the BB shell repeatedly and it works quite well, I actually use a large drill to run my "taps" in and out repeatedly.