Unless someone has removed it, there should be a metal tag held on by one of the rear end cover nuts. If it is still there, it will have a unique shape that is used to identify the ratio. When you find it, you can go onto the internet and search for Chevrolet specifications to find the different tag shapes used and what ratio they represent. My guess would be that, in as much as it is a 6 cyl. car and is stick shift, the ratio is probably 3.31:1. Another way ( I have never tried this ) is to make a chalk mark on the driveshaft and a chalk mark on the tire. Then, find a way to count how many times the driveshaft turns in relation to how far one revolution of the tire goes. Good luck

Sources: Impala Bob, Mr. 409,Late Great Chevy, Hubbard's Impalas