If you're doing a street/strip truck, it's gonna be hard to beat the 2-bar rear suspension that's under the truck now.

If you want it down a little in the rear, contact these guys. They can provide exactly the correct springs to give you the height and ride that you want....
http://www.eatonsprings.com/

If you want it down in the front, install a set of 2" dropped spindles. If you change the springs in the front for shorter ones, you just screw up the front geometry because you change the angle of the upper and lower control arms and the truck won't handle right.

If you plan to go hardcore drag racing with no street use, get Chris Alston's catalog....
http://www.cachassisworks.com/cac_catalogs.html
Also get his how-to articles....
http://www.cachassisworks.com/cac_how_to.html

Click all the links at the top of the page and read all he has to say. It will help you make a decision on what to do with the truck.