Have you read through the threads in the "Increasing your IQ" sticky at the top of the IQ forum? Tons of information on your sled.
What are you really looking for with "more stiffness" out of your shocks? Are you looking for a stiffer skid suspension or front end? The first thing I would recommend in swapping out your rear springs with .375/47's (part number below), to improve your rear suspension through it's full travel and get rid of the excessive rear sag. You can also relocate the rear scissor arm back 5/8" to remove almost all of the sag without creating a stiff ride. Then swap out all of your shocks with something better than your current non-rebuild-able shocks. For coil over shocks, Fox Zero Pro's from Carl's Cycle or Walker Evans Clickers from Tri-City Polaris are a great inexpensive option, both are available with single, double, or triple rate springs. Of course if funds are not an issue you can go with Raptors, Zbroz, etc.
7042079-067 Spring, Torsion, LH, STD [.375/47 °]
7042080-067 Spring, Torsion, RH, STD [.375/47 °]