The 6.0 liter engine is stock on the 2500HD series for the current year, unfortunately the truck doesn't come with a regular cab/short bed option. Neither engine is an option for the 1500 series so you can't get the regular cab/short bed with that engine. Both Chevy and GMC builds for short beds with the 6.0 or 6.2 engines have to at least be a 2500HD truck and you can only get a regular cab/long bed in that series. It is always possible to see if you can SPEC one but I am not aware of this being available at this time. Check your dealer.
Yes, However most dealers do not want to bother with a special order. So I would be assertive, and say "I'm sure someone at another dealership would love my money!" Then they'll think twice before giving you a run around. Happy New Year!