Clearancing the block should not be a major isse but it will be some work to do by hand. Those blocks have been clearanced for a larger stroker so if he is careful and cuts in the right places and does not cut/grind more than needed for clearances it all should be fine.
Instead of water jackets he might want to be concerned about getting into the oil galleries that run at the bottom of the block from front to rear in that block.
All it takes is to be careful and measure and check it and keep checking it a little at a time.
I have done several 396 cu in using the LT-1 Blocks. I have no idea what he means about the one piece seal making the block thinner. Also you can easily make it a block with a two piece seal with an aluminum adapter.
Ed