A couple options..
You could use shocks with "booster coils" on them...
or You could add another leaf. Ideally you would want more leaves in the front of the spring pack to help control axle wind-up (like ss springs). My suggestion if you have another set of a-springs is use the second or third leaves and put them in your springs.
However, this would all change depending on why you want stiffer springs.. Just to raise it? to help handling?
Oh! And make sure the spring hanger is correctly lined up in relation to the spring.
