Let me clear up a couple of things. The body sequence number on the trim tag is not a VIN nor was it associated with the VIN. That is a Fisher Body number that was assigned to it when the order was accepted. If the two numbers match or are close, it's strictly a coincidence.
The X33 is a Z/28 only code. Being written on the seat back support, provided it's for real, only solidifies the car as being a real Z/28. And as I mentioned before, having glue over those markings is a good sign. Otherwise someone would have gone through some clever fraudulent steps, back in the day of plenty (compared to today) when doing that kind of thing was hardly thought of.
Your 1966 block has no VIN because GM didn't start VIN matching the engines until 1967 on some models, and on all models by 1968.
There is more on your car to strongly suggest it being a Z/28 as opposed to an SS. Of course, without the numbers matching engine or documentation, you can only work off of a strong hunch based on the gathered clues.

 

Thanks guys,