The station wagon presents much less frontal area than a van as well as a much better aerodynamic coefficient. I wouldn't be surprised if the wagon weighs less than the Astro van as well. The tall gearing of the wagon probably works out pretty well also.
This is a man that obiously knows how to go about doing an engine swap.
As for the diesel comparison, once you have the MB diesel affliction, as I do, it's difficult to lose. To me the diesel stinks so good and it rattles so good. The incredible longevity of the MB diesels if properly cared for is great.
I compliment you again, Mr. Wooldridge, on a job well done.
Have a great day,