The 6.0 is as good as gas engines get. In my opinion you're FAR better off with that engine in a van than a diesel. The vans with diesels are painful to work on and the 6.0 gas engine is extremely powerful and reliable. Our 2003 GMC work truck has one and its got over 270k on it with very little maintenance. The truck is just used and abused and always comes back for more. We get over 10 mpg easy and that's pulling a huge trailer with two commercial mowers.
I'm no fan of gas buses but that's just a van with a mini bus body on it. Gas engines aren't as big a detriment on something the size and weight of a pickup truck.