Has the engine been rebuilt probably since its 40+ years old. Its common for the rebuilder to deck the block and wipe the vin number clean off. On pontiacs its more common since the cr is low a lot of people try to bring it up by decking the block my self included. So i would not be suprised to find the vin missing. Modern cars do not stamp the vin in a location that it can be removed by normal machine work.
If its a collector car. Get the engine build to sign a letter that the vin was in place before the rebuild and the deck was surfaced. Not ideal but it happens. Hopefully you can find the shop that did the work. I have also herd of some acid that will reveal the number even if its removed but not sure this will work on a 40 year old engine. works for guns i know. but probably not engine blocks.
older engine did not have the vins.
Also you are looking for the last 6 or last 3 digits of the vin. Not the entire vin number in most cases.
Some rebuilder and restorers can restamp the vin number on the block. But its shady practise and needs to be done correctly and legaly. LOL
Also factory replacment blocks did not have vins stamped on them since it was a replacement part. FYI you got the block replaced under warranty. This could of happend the first year of its life.