Farrrrrk.. its a massive job and big bucks, you need to relocate the starter and all sorts or things.
There is bugger all room for the heads and pipes ect. Trust me its a stuff around and unless your extremely attached to the car or you picked up a LS1 and box for next to nothing it wont at all be cost affective.
Done correctly, there is nearly 10k in conversion parts...
Put some turbo's on your current v8, give it a tune and have some fun with that.. Much cheaper and way more power then a mildly worked LS1....
And I know I might need to don the old flame suit for this next comment, but its actually cheaper and easier to put a 2JZ in and it will achieve a greater amount of power for much much less then the above 2 engines. The 2jz can get close to 1000hp with an un opened engine and with just bolt on parts and a tune, and it can actually handle 600hp on the stock ECU.
Video on youtube (2jz VN) is pretty cool to watch
2jz vn commodore - YouTube