The last year for the 8.1 big block in GM trucks was 2007...2007 was a transition year, the 8.1 was not offered in the new models, but they ran the old models one more year as the "classic" style, so you could still get an 8.1 in an 07 Classic but not many were built. Most of the 8.1's are 01-06. No manual trans was offered, but the Allison automatic 5 or 6 speed did have a full manual mode that enabled you shift it like a manual, just without the clutch :)
Once upon a time you needed a manual to hold up under a heavy load, and to be able to pick the right gear to control that load. Modern automatics like the Allison do it just as well, and I'm with T&P on this one, let the computer worry about whether the converter is locked or not, as long as the temp stays down, it doesn't matter.