Almost any enclosure design in an auto environment is trial and error.
There are almost too many variables that you have to factor in to be able to design a 100% perfect box.
I've built and installed CRX style boxes for many different style cars and suv's. I've never once had one that didn't preform as expected. 91 CRX, 99 Blazer, 96 S10, 92 Corsica, 89 Blazer, 84 Infiniti, 00 Venture van, to name a few.
From my experience the output using the CRX style has been low, loud, and clear. (Dependant of the tuning freq of course)
But that is only one way of many to build a good box. And everything is car dependant.
Believe it or not I got the best sound out of my trunk car using a sub that fires up, as close the the rear of the car as possible, and port back. I simply couldn't get good sound any other way. I used a kicker 10 in a small sealed box and moved it all over the trunk, experimenting with different freq responses and output levels.
I'm putting my money in the CRX box giving you the highest volume level over the widest range of sound in your van.
With such a long cabin, your going to have the advantage of a pretty low vehicle resonant freq. That alone will boost your sound down to below 30hz.
93 Honda Civic SI
Alpine CDA 9885, 30gb Ipod
Lightning Audio Storm X1 10
2.0cu ft @30hz
LA Storm 1000D
JBL P660 comps
Soundstream T4, 15