Is it a 10 or 12 bolt rear? 10 bolts were installed in Firebirds and 12 bolts were installed in other GM products. If you have a 12 bolt it's not original. If you want to know your "rough" gear ratio you could always mark the rear wheel and driveline. Rotate the driveline 1 turn, count how many times the rear wheel turns. This is your "rough" gear ratio.

Also, the donor car where I got my 68 Pontiac 350 engine had the stamp under the hard brake line on the axle. This wasn't detected until the rear axle (and brake line) were removed. He hunted for the ratio forever until he actually removed the line and there it was.