The part or casting number for the block will be on the right hand side of the block, right at the back, above the
oil pan, and below the head. It may be difficult to see it without removing the starter. Remember this is the part (casting) number, not the engine serial number. My 200 HP 289 in my '66 Mustang has the following number: C6AE-6015-C.
One quick and easy way to identify a Hi-Po 289 from run-of-the-mill 289's is the damper. The Hi-Po will have a much bigger one.
Wayne