The computer could still be flashed to a 4.8/M5 truck VIN number, then you'd just have to copy 5.3 data into the tune. Would be really simple to do that.
2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.
If you don't post your tune and logs when you have questions you aren't helping yourself.
I know this isn't exactly a definitive source but it's quick and easy, RockAuto shows a manual trans for 2003 Silverado for both 4.8 & 5.3 (though not for 6.0). No '03 manual files for any engine in the repository to steal a valid VIN from.
https://www.rockauto.com/en/catalog/....3l+v8,1412086
Here is a little "thing" I do. I have Techline Connect, SPS, GDS2, etc. I get to factory program anything I want. That being said what I do is go onto vehicle for sale sites and search for what you want. X vehicle with X transmission with X engine with X fed or Kalifornia emissions. Most will have the VIN because everyone wants to carfax etc. Use that as a VIN and bam. You're half way there.