I have my first 68 Firebird and discovered I have the same style rear you do. I pulled the diff cover and counted ring gear teeth and also discovered I have a desirable 336 ratio 4 pinion nodular safety track.I was told they are a good rear and valuable. Good idea to change your gear oil anyway and see what condition gears are in. If you have posi make sure you add the posi additive. And in my case I also discovered seized wheel cylinders and a bad axle bearing which turned into an axle replacement.