[Aussie Mike]

What's going to have a big impact here is where the bottom shock mount is in relation to the wheel. You have to think about your front lower arm as a lever on the shock. The further inboard the shock mount is the more leverage the wheel will have.

My shock mount is right out at the ball joint so a 500 pound spring is going to be really stiff in my car. Your shock mount may be half way along the lower control arm so a 500 pond spring might be OK. You really need to look at where the shock mounts are in relation to your pivot ponts and the point where the wheel hit the road and work out your "wheel rate". This is actual spring pressure at the wheel which is your spring rate multiplied by the ratio of your shock pivot position.

Another factor is the angle the shock sits at. In a vertical position the spring rate will be 1:1 but if coil over is mounted at an angle the spring rate is effectively reduced.

The small diameter springs for these coil overs are relatively cheap and very easy to change out. So you could run a sellection of springs. 500 pounders for the track and 400 for the street.