Of the choices you listed I'd take the 8.1 Sub. I had a dually with the 8.1 and it's a great engine: smooth, quiet, powerful.
Have you considered a Duramax Suburban? There are several companies around the country that make them. Duraburb is one of the more popular ones, I think they're in Florida.
I saw one at the Marina here in VA a few years ago. I asked the guy where he got it. He said a company in Ohio built his. It looked totally stock, right down to the emblem on the door.
Do some research on a Duramax Sub. It might be just what you need.
I know you said no vans but 4 x 4 vans are really cool. I've seen two Chevy Duramax vans with Quigley 4wd under them lately. I wish I had a reason to have a 4wd diesel van.