Note that the van is an LT40 commercial van, not a smaller passenger van. The 1st-gen LT40s were available with an I6 Diesel direct from VW. The 5.9 is a bigger beast for sure, but its not an inconceivable swap.
I know this is an old post. FYI, I know this van and it does actually have the 5.9 cummins engine.
Clearly not standard, it was converted a couple years back.
It's a 4x4 and was running a 2.4 Straight 6 TD before it died.