You want to give that rear spring its due respect because it can be lethal. People have been injured messing with a rear spring and not realizing the tension packed into it.
The safest way is to obtain a proper spring spreader, one that hooks against both shackle bosses. Have the rear axle housings sitting on jack stands and put a floor jack under the banjo. Spread the spring sufficiently to be able to remove the shackles. Lower the rear end down with the floor jack. Collapse the spring spreader and remove it. The rear spring is now safe as all the tension has been removed. Unhook the two U-bolt assemblies and remove the spring.
Install the spring in the reverse manner. Be sure that the square headed center bolt slips into the mating hole in the cross member. This centers it and locks it in place. It helps to put a chalk mark on the cross member and at the center of the spring under the bolt head.
Once you have the spring secured with the two U-bolt assemblies put the spring spreader back in and crank it out to match up with the shackle bosses on the rear end.
It is not safe to remove a rear end from a car with the spring still attached, even though people do it all the time. The spring is under a tremendous amount of tension and the only thing then holding all the tension is the single center bolt that was not designed to hold that much tension. If it happens to be 80 years old and rusted it can let go and allow the leaves to fly 20 feet in the air and take your head with it if it is in the way.
Work safely
Tom Endy