Here's the casting number on Paul Calvisi's '65 289 High Performance block. But don't get excited if you see the C5AE-6015-E casting number behind your Mustang's starter. It doesn't mean you have a Hi-Po block. It means you have a 289 block. What makes it a Hi-Po block is its wider main caps. This C5AE-E block was cast on February 25, 1965-date coded 5B25.