I was under the impression the LS1 was alloy and the LS2+ was cast iron?
The LS1 motor is lighter then an RB motor, but the gearboxes are heavier, providing better weight distribution.
You will need custom engine mounts though, but nothing too difficult.
I've always wanted to do this to an R32, but keep it NA. Nothing more embarrasing then having you arse handed to you by a non-turbo Skyline