FYI.... We are just above 170mph at Daytona... Sooooooo unless you are tackling the salt flats there isn't /shouldnt be a problem.
Easy swap go to :
www.car-part.com search and purchase a rear carrier / housing from a manual Z from salvage yard. When going that route...there will probably be less of a chance at needing to shim the diff unit itself when swapping gears...giving you a better line up on the gears and less gear noise.
Plus when selling the car etc... Makes putting It back a breeze.
Hard searching you can usually get them for $500 shipped. (entire complete rear carrier) you will have to use a little elbow grease when it arrives as the outside of the casing may be a little site to the eyes...but it's what's inside that counts.
The viscous lsd's have one half shaft that is slightly longer that operates the viscous unit. When switching to a clutch type LSD... And I highly recommend OS Giken unit.... You will need to purchase a Left side half shaft from Nissan.( shorter of the two shafts)
Any guys seriously tracking the cars ...or not. I would seriously recommend sending the 4.08 gears to be micro polished.... It can cut down rear end temps by about 15-20 degrees... Doesn't sound like a lot....but it is with how hot these puppies run.