Would love to see pics of your friend's vans!
Destroked did b. rock's conversion and I heard they wouldn't do another van. DCS did a 5.9L common rail in an Eseries but wasn't interested in providing any parts or information about swaps in vans when I contacted them.
Vans aren't nearly as easy to Cummins swap as the trucks so be aware of that before jumping into it. Totally possible, yes. Totally worth it, yes. Don't underestimate or minimize the amount of work though.