Coilover shocks on Model A P/U question

I have installed a 4-bar suspension on the rear of a Model A pickup, and plan to use coilover shocks. I'm hoping someone with some experience can suggest what spring rating I should be using. I've been doing some reading on web sites but still don't know how stiff the springs should be??

 

Light PU 160lb

 

I have 180lbs on my A coupe

 

I think mine are 180lb. too.

 

the rate will also depend on if they are vertical or leaned over. increasing angle decreases the effective spring rate.