Well, I wouldn't take this thing on anything rougher than a firm dirt road, but how far afield you're willing to go is a personal decision. But belly pans are never a bad idea, and if you attach them with cable ties, accidentally losing one on a trail is a simple matter of a few new cable ties for reinstallation. As I recall with the Van, the underside is a nightmare of greeblies and bits in the wind, so you could achieve quite a bit with a belly pan and not alter the appearance at all.
The engine is under the front seats - which made me raise my eyebrows when you described the vehicles as "practical," but I'm judging that statement from a maintenance perspective - so you don't want to belly pan the whole thing. But everything from the bellhousing aft is probably a good place to add some smoothing work, and there's also quite a bit you could do ahead of the front axles, closing in areas that don't need circulation. Just so long as the radiator can breathe, you're good.
Adding a minimal boat tail is completely feasible; I can't remember the member but his Previa is called "The Golden Egg," he added some small kammback additions to his ride (in stainless steel if I recall correctly) and he's getting better mileage than I am. The blockier van you're using may respond more positively with any aero improvements.
This would require some A-B-A testing, but I wonder about adding a bit of a forehead to the top of the van, so you could bring in some taper to the roof as the roofline moves to the rear. That's just spitballing, the added frontal area may completely offset any wake drag gains.
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Lead or follow. Either is fine.
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