If I can run a 13.7 in my 97 which had less HP than your 5.3 at the time, you should be able to get in the high 13's, or low 14's with the right components. Gears make a big difference, I had 3.42's and went to 4.10's. Sure I lost sme gas mileage but the OD tranny still helped out. Are you running 3.42's? I was spraying the old Vortec and was able to take most supercharged or turbo trucks before I broke a valve spring. The truck weighed in at 5000 lbs with just me in it.