The 6.0 will pull it, you will know it's there. It is a good motor that been out for a long time. It will make power but need to rev to do so. If I was towing with one I'd want the 4:10 gears.
The new L5P Duramax will pull 10K like it's not there. The Duramax has also been out awhile, and mechanical part is also proven. There have been software and sensor issues, I don’t know if GM is having any more or less than any other manufacture is. Our 05 Duramax never had any major issue, nothing internal. Our 18 is quieter and rides better. Time will tell if it’s as reliable.
Bed length is personal choice, We have a 5’th wheel and I prefer the 8” bed. But the standard bed trucks will be easier to park, and maybe fit in a garage.
