For the racer that is running a competition block mounted idler bracket on there blown BBC these will space out form the engine block one quarter inch and they allow a sturdy foundation to mount the idler bracket off.
Water blocks use the standard .375" two bolt mounting pattern and fit most all factory BBC and aftermarket Donovan, World, Dart, CN, Rodeck engine blocks. Water blocks are made from billet .250" alum. with a Buna o-ring to seal the water block to the engine block.
Blocks are sold separate so if ordering remember you need to order two for a complete set.