the 12 bolt rear was already installed in my 68 coupe when i bought it. According to previous owner, the 12 bolt and shaft were from a 68 or 69 big block Camaro running a 4-speed muncie. If you do the 12 bolt, make sure you know what series carrier you have. a 3 series will take up to a 3.73 gear. you would need a 4 series for a 4.10. As long as you know what carrier you're running, Richmond Gear carries the thick ring and pinion gear set for the 3 series carrier, if your stock gear is, say, 3.23, you'll need the thick 4.10 gears for your 3 series carrier.