The first time I saw one of those blocks was at Kenny D's shop about 25 years ago. He had led me to believe they made 50 or less. The backstory he gave me was that they had been intended for use in a TransAm race series of some sort that never got off the ground. He said they were sold by GM as an Olds V6 block. You're description is accurate, they are basically an on center block with the pan rail cut off. They will require a custom oil pan but that's not a big deal.