Upgrade for '70 Swinger 340 rear

Super Stock springs are designed for the drag strip. They are an attempt to get the car to launch flat so when the car is just sitting or driving down the road one side in the back is higher than the other. You can get A-body 6 leaf springs from ESPO that will work a lot better for driving around (you said street car, right?). Do you (or do you want to) drive the car out on the highway at all. Sustained driving over 50 mph with 3.91 gears is torture to me. IMHO 3.55 is as low as I would go. If it's really a street car, tapered bearing all the way.