[BigAls87Z28]

There is no difference or any hp gains you will get by going to it. The LS6 block was made in Mexico, the LS1 block was made in Canada. The LS6 used a different process on casting the block to make it stronger. The Mexican plant used to produce the same LS1 style block Canada did, but once they were told to make the LS6 block, some were sent to the Fbody plant.
In 2001, about 15% of the Fbodies out there have an LS6 block, and by 2002, up to 25% of the Fbodies have the LS6 block.
In 2003, the Canadian plant went to the LS6 design and casting process, so by then, they were all LS6 blocks.
The LS2 has taken most of the technology learned by the Gen III engines and pushes them to the next level, so on with the LS7.