dont really know too much about lowering but as far as the 03 6.0 goes I could help. Pull off that stock exhaust and ditch the muffler and cat totally. You can get yourself an intake or even an after market replacement. If you want to have even more fun go to
::: DJ's Perfromance :: Home ::: order yourself an SCT and buy some gauges to keep an eye on everything. you'll have alot of fun and be super impressive with the power out of the 6.0. I know I was!