[elhigh]

I like the 300 and wouldn't change it even if I could, were it me making the decisions. I'm a sucker for old school tech, and a stout truck six like that is right in my wheelhouse. It isn't sophisticated and it isn't especially powerful, but it is dogged and relentless, and that appeals.

The big clunky hinges are an advantage in one way: they provide ready-made mounting points for your tail if you decide to add one. If you can get the hinge pins out, you could replace them with slightly longer ones and install tail panels on those hinges, making it so that you could fold the tail flush against the rear of the vehicle for in-city use or garaging. You could even feasibly still open the doors partway without disturbing the deployed tail.

The underside of your van is an aerodynamic mess and you will reap benefits either from an airdam as you've already discussed, or from adding a belly pan and skirts. It might also make the vehicle quieter, but I'm only guessing there.

I remember working under a 15-pass I used to drive for work, once I was under the vehicle it opened up and there was quite a bit of space in places, next to none in others. Very craggy and uneven.