Those are two different tires you're talkin about. The 325/50 drag radial has about and inch more section and tread width than the 28x12.50 ET Street. If your car sits at stock height and you don't mind the tire being close to the fenderlip, then you can get away with a notch just past the seem, crossmember side to side inbetween the shocks and springs, and some BFH clearancing of the factory wheel tubs. If you want the car to sit low then you need a full notch to the inner skin with bracing on the inside of the frame rails, crossmember from side to side running inbetween the shocks and springs, some additional bracing and a crossmember near the rear body mounts, and fully remove the factory rear tubs inner and outer, install 35"-36" tubs. Also would be a good idea to do at least a 6-8 point cage tied into the crossmember near the springs and the upper and lower control arm mounts. If you have a stock gbody length rear end a 5.5" backspace will work fine, if you have a 68-72 chevelle length rear end you will need 7"-7.5" backspace.