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The spring should have a bolt that fits downthrough a hole in the attachment plate that will ensure that the spring is centered.
I am assuming that you are putting the spring on the frame with the body off. Take a tie strap-large-and hook one side to a jack and then run it around the frame and hook the other side to the other side of the jack. then ratchet down the strap to take the slack out. Now start jacking underneath the spring (you should have aligned the jack under one side or the other before putting the strap around the frame). the straps will keep the frame from lifting and you will be able to put the bolts through the spring holes.
Good luck
DO NOT fully torque the four bolts through the spring retaining plate until the full weight of the car is on its wheels (spring flattened out), or you'll probably break the rear ears off the rear cover. If the car isn't on wheels, just tighten them snug (15-20 ft-lbs.) and fully torque them later.