About using "C" clamps...
A couple of years ago, I think it was Miss Shelly posted a picture of a C clamp that had been used to clamp a spring together. It was a big clamp and it was pretty much "closed" when an event occurred. It seems that for what ever reason, a small amount of force was induced perpendicular to the axis of the threaded rod of the clamp, putting a lot of torsion on the "C" frame of the clamp. The frame twisted a little which increased the torque and everything went to hell. There's a lot of energy in a compressed spring which can do a lot of damage. Personally, I use a 3/4" Pony pipe clamp to hold the spring while I remove the center bolt. Then, I insert a piece of threaded rod and back off the rod and clamp alternately. Everything is nice and close when the clamp is "closed" and you don't have the large twisting forces. I reverse the operation to put the spring back together. If you're using a spring with reversed eyes, you'll have to use the clamp / threaded rod method because there's nothing for the typical spreader to grab onto.