400 Cylinder Walls Breaking ???

We have had a few cylinder problems with the 400 Blocks

We now run exclusively the 400 Cold weather early blocks out of medium
duty trucks. Easiest way to tell is the early casting # PLUS casting number is TWICE
as large on the side of block.

Medium Duty 400 Blocks???
What part number/s where those?
Are large of a bore size are you running?
I would like to see a photo of the block sides.

Thanks,
Tom