LS3 vs. 408ci
#2
The 408 has more cubes so it will be able to make more power. I dont know the price of a new LS3 crate motor but I would imagine you could get a 408 and the necessary parts for cheaper.
#3
TSP has options for a 418 and a 427 cid LS3. look on their site for prices i cnat remember off the top of my head
#4