First off, the car came with a 7.5 inch rear which will not last long behind a 400, especially if it ever hooks up. You`ll need to upgrade to a least a 8.2 at a minimum, if this car is to be a daily driver then I wouldn`t recommend gears any lower than 3.42`s, and if you set your 400 up to make lots of low end torque, which 400`s excell at, you don`t need a lower gear than that. if you went 3.73 or 4.11 it would be winding around 2800 rpm and higher at cruise speed which will eat a hole in your wallet at the gas pump. I would stab a cam in it with 224 duration @.050 and add a double roller timing chain with a set of matching valve springs, get a decent rear under it, add your 2500 stall converter, tune the carb and ignition to take advantage of your new cam, take it to the strip and see what time it runs. I doubt it`ll land you in the 12`s, but it will give you a baseline on what mods you`ll need to step up to in order to reach the 12`s. Adding a set of decent heads on it would also be a step in the right direction if the heads on it are stockers. But keep this is mind, don`t get rev happy with it, 400`s aren`t revvers, and with the torque they make it don`t need to rev to get you moving, I wouldn`t rev it past 5500 rpm, also keep a constant check on your oil.