Slant 6 mod's

When I purchased the car from the seller near Albany, NY in a town called Broadalbin, NY, the seller said that it had a 3:08 rear end ratio. However, there is not a tag on the rear, and it is a 7.25. I am looking to improve street performance, and I am not into drag racing.
If the tag on the rear is gone than I guess it is impossible to know the ratio?