I have the same 2002 2500hd 6.0 and what I did to it was build my own cold air using 4" stainless tube, 4" velocity stack, 7" inlet cone filter and some sheet metal for a box and lined it with heat insulation and opened up a path for air to go between headlight and rad directly into filter (made small gains but gains) installed headers and dual 3" x-piped exhaust into a dual in and out 3" mbrp straight through muffler Yed into a 4" mbrp resonator and 4" over the axle to 5" tip (again small gains but really good sound and cruises at 67-70db) and finally I got my computer tuned (not a plug and play chip) the tune did the most for me, better fuel econ. And a lot more power and changed the shift points to be make it faster, if you do get it professionally tuned look into what the trucks doing when cruising (assume when torque converter locks up) I had him add about 7* of advance to the factory map which is why my fuel economy went up (20L/100km to 14L/100km or 12mpg to 16mpg)