thicker cylinder walls. The reason that guys think that they are thicker is that Ford improved the core setting process for the MCC plant. So since they have less core shift you can usually bore them out to a larger bore size before getting too thin compared to the DIF blocks.
Like Tom I also raised my eyebrows at the "minimum" .280" wall thickness. The only factory block that could even come close to this is the super rare FT 4.05" block cast with 428 water jacket cores.