I looked at some fuel tanks and at myb truck along with others... The truck fuel tanks are taller in the rear area where they fit under the bad, while the van tanks are a little shorter to fit under the van floor...

I've got the 35 gallon version of each sitting in the garage. The truck tank came with a lot of things I bought together and I picked up the van tank from a local salvage yard. I'm going to see if I can shorten it before going the custom tank route... Thanks again for the input / thought!