Some older Ford vans, up until 1996 I think, came with the 4.9l 300 cubic inch inline 6 engine. It is one of the best engines Ford ever made, ultra reliable, good gas mileage and very good low end torque which is great for mountain grades and slow crawling on forest roads. Also the absolute easiest engine to work on in a van. See video link to see what I mean, no van engine comes close to being this easy to work on.
Ford Van inline 6 video
I had a friend who owned one and he loved it. My father owned one long ago and we camped in it all the time.
They are not very common to find, they are not fast, but extremely reliable and will go up any grade. Check out the video I put it at right where they look at the engine compartment.