Factory Nissan R200 is good up to around 750whp and is the upgrade to the R180. The only problem would be fitting the proper final drive ratio, but I believe that an Infiniti J30 final drive is something like 3.90...

The custom axles I got made from Moser still use the Subaru hub ends but the shafts are cromolly. The shafts are what were breaking on my RWD conversion, but I haven't broken anything since the conversion (which I hope I can still say a long time from now).

If you're interested I could find out more info on gear ratios, subframe designs, getting my axles duplicated, etc.