You need to measure the travel at the shock mount extended/compressed and select a shock length that is outside those two measurement. Make sure the shock isn't the limiter.
Since you have what appears to be a ride height lower mount set up, you can play a bit with that. Get the wheels/tires under it and set height where you want and measure away.
As far as springs, I'd start with a 130# and preload it a bit more than say a 150 with less preload. Rather have the stored energy in the spring. The ride would be a bit better, less harsh.
Maybe this will help.
http://www.cachassisworks.com/Attachments/Instructions/899-031-226_drag.pdf