The LSX is Iron but it's a different animal to the 6.0 Iron truck block. It's got siamesed bores with a lot more material in there to easily take a 4.25" slug in there. It also supports 6 head bolts as well as the regular production 4 head bolts per cylinder. The other main thing is that it will support upto a 4.5" stroke crank. Combine that with a 4.25"bore and you have 511 cubic inches.
Cheers