To clear things up a bit, you need to understand that the trim tag on the firewall is not a GM piece but rather a Fisher Body piece that denotes options on the body of the car from the firewall rearward. The cowling and trim tag are among the very first pieces assembled when the car first started down the line and was used in conjunction with the build sheet (Spread Sheet) for assembly reference. There is a gob of info contained on the trim tag that has no reference to the Body's VIN number tag located on the driver's door jamb.
The VIN Number typically has about 5 digits that denote the body style and weather or not it was originally a V8 or 6 cylinder car. These do not tell what displacement or horsepower it was. The letters that show in about the sixth place of the VIN denote the assembly plant (ST would be St. Louis, B for Baltimore, D for Detriot and so fourth). The 4-7 digits at the end are the sequence number for that body style from that assembly plant.
A typically trim tag contains the info for the month and week of build, the paint codes, interior code which will denote cloth or vinyl and colors. Some paint codes will also have coding for a vinyl roof or convertible top color. Since the SS cars used a different interior trin code, it is easy to tell if it originally had the bucket seats and console.
The tag will also typlcally have 3-5 codes at the bottom that could denote several things. These are pretty hard to decipher and some can't be at all. These items would be for things like tinted glass, air condiotining, right side mirror, padded dash, etc.
I would think the best you could hope for would be to make note of any options you think the car has and try to go from there. Youll at least know what assembly plant, and most of the factory options.
To shed some light in another area might be to look for a 3-4 core radiator and 3/8 diameter gas line instead of the normal 1/4 inch line. If the car has this, it would have originally been a big block car. Does the car have only one or two upper trailing arms for the rear end? Cars with more than 250 HP had both.