Not sure what kind of budget you're looking at...
The LS7 and C5R blocks are built to handle more power than the other factory GM LS aluminum blocks. If you want to go aftermarket and have a big budget, Dart and RHS offer aluminum LS blocks that will handle well over 1000hp without the weight penalty of iron.
I've seen guys run big numbers with turbos on iron LS truck blocks. 5.3 and 6.0L variants are readily available and usually pretty cheap. If you're on a tight budget, this is likely your best option.
The LSX block is iron and is rated to 2000+hp. It's heavy, but it's not going to fail.