Probably a good idea to check the more drifting orientated forums.
One comment though, with those spring rates you certainly will need a full 'roll cage', probably more like a tubular sub-chassis, if you get my drift, pun intended The 240Z has a relatively flexible body, springs at those rates will just force the body to flex. So you are going to have to strengthen it, not merly install a roll cage.
One of the reasons why the Silvia makes a good drift car is its stiff body, they make a handy race car for the same reason.
On the subject of shocks, check with Koni.