It is easier to get an aluminum block than a cast iron block. The Bill Mitchell block is aluminum, when it is available. He used to be associated with World and still is, a little. But he took the aluminum block line and left the cast iron block line with World.
From his comments about improvements in the machining processes he was moving to on his aluminum block, it sounds like World had some worn out machines that couldn't hold tolerances.

I don't know anyone turning out new cast iron big blocks at the moment, except for Chicago Connection, which is only a maybe.

R.