Casting block question

[Kw69]

Hey guys, hope everybody's builds and projects are going good. I may have asked this before but the original engine that came with my truck has a cast number of 3956618. I heard that this block was only used for a few months and for just the 1969 series. I was also told it shared the 350, 327 and 302 internals depending on what vehicle it was going in including the famed camaro z28. The engine was pulled out and a new drop in was installed by the previous owner who has passed away. When I bought the truck, he said that was the origainl engine but had been rebuilt somewhere years ago from the original owner. Unfortunately I can't see the vin cause the deck has been blocked. I did find that this engine was made in 69 cause of the codes. I have a few questions though: is there any other location of a vin number? And is this a rare block cause of the casting number? A friend who has raced cars said that it was a rare block and it was a desirable one along with the high nickel content. Just figured I would ask. The engine right now is sitting in my shop. It's complete and I plan to open it up in the fall. My desire is to put it back in the truck after a rebuild if it needs it. The previous owner removed it because he said it smoked a little bit had no loss of power. So it may be something simple like valve guides, or seals... Who knows. Anyway thanks for all input.
Kris