Official documents sttaing it was built as a lwb combined with the correct serial# on what appears to be an unaltered swb frame is the stumper to focus on, as the Op already knows. So that's where we'd have to look at the possible changes.
1- Maybe the guy paid GM to falsify the record... I agree, far fetched wildass guess
2- Frame was shortened so well it can't be detected... Could be.
3- VIN was sectioned in... Very possible
4- Front end clipped...Also possible but harder to hide. Same as #2
5- You need to get your eyes checked?
just kidding
6- That truck is so nice this is actually a dream.
Anyone have a better 5 or 6 or 7, 8, 9, or 10?