Congrats! You have a true 71SS!!
Although I do not have a build sheet for my matching numbers 71SS, because it says Z27 on the trim tag and I also have a CJD block I can say beyond a resonable doubt that yours and mine are both ture SS cars.
Regarding the VIN, I would not worry about it. I would still consider it "numbers matching" or at least "original motor".
My situation is similar but with the tranny. The VIN is off by one number. The dash, door sitcker, and motor on my 71SS show 1N502145 but the tranny shows 1N502146. See my post from a few months back when I had the motor and trans out:
The experts on here determined to be a mix-up on the production line yet I am still considering the tranny to be numbers matching/original.
Have you checked your tranny number yet?