"The LS1 block is made of 319 aluminum heat-treated to the T5 standard by the Montupet Corporation of Ontario, Canada. It is cast using the semi-permanent mold technique which Juriga described as "....a cross between die-casting and sand-casting." The case weighs 107 lb. Compared to the Gen II's 160 lb. block, that’s a significant weight savings."
This qoute was taken directly from an excellent online article you can find
here.
This makes sense after viewing the LS1's water jackets, which are very open but still a closed deck design. If it were truly die cast the deck would have to be open. I can't currently upload pics to the board, so you'll have to find your own pictures online of the block sans heads. Hope this helps, peace.