The Cali thing who knows with your laws? Yes if there is room you can add a fuel cell like they make for race cars. Look under the van, I'd position it so its protected by and mounts to the frame. http://www.summitracing.com/search/part-type/fuel-cells
We had one of the stock car builders in town make us a custom tank which we mounted under a diesel box to fuel out gas TM.
Do you know who sells those? and which vans (Ford E 350 or Chevy 3500 1 ton. Both regular size- non extended) are more likely to have enough space under to accommodate such tank?
We put a 25-gallon one but it was a box truck so there was plenty of room under there for any size you wanted. It just added to the weight you carry so we kept it down but functional.
I think I'd call those folks and see if they have any idea about size if nothing else they can give you the dimensions of the smaller unit they sell.