At the time your Buick was manufactured, the company did not build "numbers matching" cars. The serial number relates to the location the car was assembled (first numeral) and the sequential order of the build. Engines had a number stamped on the block without regard to the chassis serial number.
There may be a way to associate the two numbers that satisfies the necessary authorities if presented with this information. Also, if you can contact the party who sold the car to you, he or she may be able to provide the necessary information form documents used in the the importation process. So far as I know, California registers cars by serial number, not engine number. (I have not made a transaction in California since 1969).