Originally Posted by Crazymonkeymech
Wouldnt this make it really stiff?
Yes, just like adding coilovers with the leaf springs or adding air shocks will.
That's why rearching the spring is really the only proper way to do it and maintain the designed ride quality.
If you are drag racing the adding of a second main leaf like suggested will often work well, but you sacrifice ride comfort. It is sometimes also done where only the front half of a second main leaf is added, cut off a couple inches behind the center pin. This stiffens the front half of the spring to control the spring wind-up that results in wheel hop without impacting the ride so much.
Basically with leaf springs the front half of the leaf spring pack locates the axle and transfers the thrust forces into the chassis, the rear half controls the ride quality.