Check out post #6, You can get leafs from Tamiya/USA in various lengths for the F350 just remove the rivet on your stock spring an add what you want, replace the rivet with a bolt. I had people tell me that would be to stiff but I added 4 leafs to the steer springs on this build and was still able to stuff the tires into the fenders with a fully loaded box off road.
http://www.rctruckandconstruction.co...ead.php?t=9138