1. Extending the spring length doesn't necessarily make it softer. The key is spring rate, and coil-overs tend to be linear. For example, 225# on one of your springs would compress it 1". 450# would compress it 2", and so on. Therefore, you won't gain much of a softer ride by adjusting all the way out. It will help some - but not as much as you need.

2. The adjustment only affects rebound. If your springs are way too stiff (which they seem to be), adjusting the rebound won't help much. Turning clockwise stiffens the rebound, by the way.

3. Yep, you should get some give.

4. I think your springs are too stiff. The best way to tell exactly is to weigh the rear of the car. Then you use a spring will compress about 40% of full travel under that weight. My experience is that Speedway's chart is a bit heavy on the spring recommendation. Only guessing, but I think 140 would be too light. I'm guessing that 165 - 185 would work, but that's only a SWAG. If I remember correctly, I had springs in that range on an all-steel 30 Tudor sedan.