here is my 2 cents. Getting into the 14's is fairly easy, but do it one step at a time, so you can map your progress. You will find my mods and time slip listed in my signature. A tune is a must, whether you go with custom or Hp Tuners, they are kick *** fun. This is a great site for help.

 

14's only took me a Hypertech and FIPK,(14.8) that is it, no exhaust, nothing else. i have the same as you, rcsb 5.3

if i were you, get a custom tune, fipk and longtubes, pulley. that got me to mid 14's.

 

The best thing you can do is to lose the spare tire and swap the rear bumper with a roll pan.

100% stock I got 15.220, but this was the best of at least 20 runs at the track. The average was in the 15.35 range.

My truck has the spare tire and stock bumper and with the tune only I got under 15 sec.
With the Electric fans I dropped to 14.901, and now I'm taking off the spare tire and bumper, and added the Volant CAI (cold air intake). I may see 14.5 sec. range. maybe.....

Do you have a locking differental (G80 code in the glove box)?
That would help to stop a one wheel peel, and give better traction vs. most open differental cars.

 

Something has to be wrong with your tune!. My old 4.3L 6 cyl. ran 15.8's with CAI, PLUGS-Wires, Y pipe mod,flowmaster, nitto dr's and Hpp3.