I agree with the other guy that the 350z diff probably isnt worth your hassle. Especially if you've only got the centre.
The viscous unit is not that great. On my car, sometimes it felt like it would initially lock up both wheels, then go single spinner, then lock up again if i gave it enough.
I had a proper 2-way LSD on my car for a while and it was MILES ahead of the factory viscous unit. Noisy as all buggery though.
The ATS Carbonetic centre, with the carbon clutches, seems like the ultimate option. Been in a car with this and it was quiet. Haven't been in a car with a quaiffe but it also sounds like a good option