Rear coil over shock spring rate

you need to know the weight of the rear of the car then call the shock maker. This will be impossible without four corner scales. There is another way to get you closer. btw 150lb spring sound way to stiff.

Use these two links to understand the concept

https://www.cachassisworks.com/Attachments/Instructions/899-031-226_drag.pdf

BaselineSuspensions.com.........Launching A Drag Car