With the axle under the springs....that becomes a lift kit.
The axle needs to be on top of the spring for this to actually be a lowering kit.
TVPitts...yes, putting the axle on top of the springs is the next logical thing to do. 5" of clearance between frame and axle will be fine if you never plan to haul a load in the bed, otherwise you'll need to "C-notch" the frame over the axle for more clearance. Very common with the lowered truck crowd, there are kits or you can make your own and weld it in.
You may need the C-notch and a set of 2", 3", or 4" lowering blocks like the TechInspector posted to get it low enough to look good with the front height.
Another idea that may interest you rather than lowering blocks is having the springs de-arched.