Suspension Lifting 101   A work in progress......

This shows the layout with kit supplied pieces, assembled per the instructions.Here is the same suspension with a longer rear shackle installed.
Here are longer shackles front and rear.Even more height can be gained by simply moving the spring to the top of the axle - in 1:1 parlance, a "spring-over" job.
This one has shorter shackles, but a spring-over and new leaf springs with more arch.A spring-over, arched springs and a "lifting block" between the spring and the axle. More height could be gained with longer shackles too.
A representation of a "typical" kit leaf spring.The goal for the new scratchbuilt spring.
Parts needed for making a new spring. Styrene sheet and aluminum or styrene tubing fit the bill nicely here.Cut strips of sheet styrene the width of the kit springs. Start off making them all the same length. You will need at least 20 strips for an "average" truck 
A couple of small clamps will be handy right about now. Clamp your new main leaf to the front mounting point on the chassis. Shape the spring to the desired arch by sliding the back of the strip forward.As you slide the strip forward, it will start to arch.
Bringing the rear of the spring into position will establish the length of your spring. Mark the length on the springs.The arch can be put into the springs by drawing the strip over a piece of tubing or rod. A few passes between a rod and a table or other surface will establish a permanent arch in the plastic strip (now a spring).
Cut each successive strip (leaf) a bit shorter than the previous one and "curl" them to establish an arch. Stack them up (4, 5, or 6 to a stack should be good) and glue them together carefully, making sure they are aligned side-to-side and lengthwise.Using thin narrow styrene strips, foil or aluminum tape, add "clamps" to hold each set of leaves together. This also holds each leaf to the pack.
Before fitting them to the chassis, attach a piece of styrene or aluminum tubing (rod works too) to each end of the main leaf. This will represent the spring "eyes". Adding this touch to kit springs also adds an extra bit of realism. 

It's really a simple process to scratchbuild leaf springs, as well as add some extra height to 4X4 suspensions. To jack up any leaf spring equipped model, adding longer shackles to the rear spring mounting points will work nicely. It is prototypically accurate to lengthen the shackle at the rear, rather than the front, of the spring.