If it’s there but hard to read you can try carefully to take a scotchbrite pad and clean it( I did it on my ElCamino). Also/or it you can dampen it the number might show better (have done it on fender tags).
If you haven’t yet, look under the bottom of the rear seat and see if your lucky enough to find the build sheet! Dixie Restoration, Mike’s Montes, SS396.com, Ecklers and a few more may have some of the stickers. Also check Hemmings motor news. A lot of people forget about them and it can be bought in book form.
Check with a GOOD and Reputable body shop for some answers.