Do what you have to to make that shackle angle veer towards the rear of the car. Straight up and down is not recommended because it does not allow the spring suspension to work correctly (not raising/separating the body and planting the rear end) thus unloading the rear tires.
I also love to try and keep it stock. Some cars both on and off this site have achieved some good short times using leaf springs (SS or not)
I run the Trip-city springs on mine. Just a stiffer and thicker spring leaf/s compared to the current TOY springs they now call SS springs.