Ok, lets suppose you have an LS6 block on a z. What advantage would you have with that block compared to an LS1 block.

 

Slightly stronger, but not by much, should have less issues with high rpm oil consumption related to oil misting in the crankcase (that's the theory, but some Z06's still burn oil like the LS1) and a few extra horses from reduced windage (that difference is probably lost in the range of output HP from the factory).
http://www.c5registry.com/Documents/ls6/page3.htm