well, for one, it's not positraction - it's limited slip. It's possible to replace the gears with a different ratio but personally I don't think it's worth the amount of money it'll take to do it for the results you'll gain. You can get a 4:11 rearend from an 84/5 turbo 200sx but it won't be limited slip. non-turbo z's have a 3.7 ratio which is slightly better than the 3.54 that your 87t came with.

Take a look at your differential - if it has an aluminum finned cover hten you have the LSD one, otherwise you have a regular open diff.

240sx's came with lsd 4.11 rearends i *think* but the gearbox is different. I've heard you can use the 240sx rearend along with halfshafts from a z32 for that type of setup. I'm not 100% certain on that though, so maybe ask around or consult some factory manuals/specification sheets first.

 

both will lock, but only after enough wheelspin from one wheel. when it sees that one wheel is spinning faster than the other, it engages the clutch and locks the other wheel too.

positraction is usually meaning that both rear wheels are locked at all times.
i havent done the 240 tb yet, but from what i hear since it's one of the cheaper mods you can do and gives better throttle response i would do it. i already have a few of them, just havent' had the chance to install one yet. it will work best if you port the intake collector to match the increased diameter, otherwise you're opening it up and then closing it right back off again.