I have an 03 5.4 with a k&n cai edge evo programer and a custom exhaust from where the 2 sides come together it is 3" mandrel tubing out in front of the rear tire and I was wondering if I were to put a glasspack or muffler would I see an increase in power or not also what would a set of headers do to my truck.

 

say no to glasspacks. Put a magnaflow on there. You should get a set of dynatech long tubes. I gained 30hp and 35ftlbs from them. dont bother with shorties, ive had 2 sets of them, didnt gain a single hp or torque...

 

Long tubes and gears and you will be very happy.