Re: I bought a Rolls Royce and want to build it.
I'm drawing on hazy motorcycle knowledge and pictures of LSR cars with similar setups.
It's probably best if you set it to match the design action of the shock. Shock valving can be configured from the factory to act in any number of angles and ratios.
Presumably, car shocks are designed for direct action, so you should probably keep the ratio linear (1:1). If, like on some motorcycles, there is a rocker arm in the mix, you should probably try to match that ratio.
Spring rate is of least concern, because they're easily swapped for something else.