You can probaby get more bang for your buck by adding a cam and turbo/supercharger to your current 5.3L. 02Denali is making more than 300hp at the wheels with his 5.3 and a 4l80 without FI or Nitrous. Seems like it would be a shame to pull a 5.3 with only 30k miles.
No more trucks, but this is what I used to have. 2000 GMC Sierra, 6.0L-LQ4, Built 4L60, LS6 Cam, The Other Guys Headers, 3 inch exhaust and 7 chamber flowmaster, 43lb injectors, Nelson Tuning, Drop - 3/4 McGaughys Spindles, Bell Tech Shackles & Hangers. Best time 13.59 @ 106