Dirk I have run a seperate 12 gal boat tank for the last 3yrs. The van really reaks of gas. Still does after my fuel tap but not as bad. I always had to fuel the truck from both sides. That was a pain, my tank was at the side door.
Secure whatever you put on the inside. Is there not another dist even remotely close to do the tap right?
I did my fuel tap with stuff from lowes for under 30 bones.
I have a custy with a dodge sprinter and Bluewave. He purchased an aluminum fuel cell from www.summitracing.com for around 200 bucks and then had a custom made fuel neck installed and run to the outside wall so he could fill and have it vented. No problems with leakage and it's DOT approved.
I have a 13 gal boat tank because I run a trailer, only thing I don't like is it runs out of gas sometimes in the middle of the job, I keep a 1gal can with me as a spare and hopefully that is enough to finish the job, sometimes its not and I have had to leave in the middle of the job to get gas
For some, getting the fuel tank (gasoline) is the easy part. Most every fuel cell/tank builder here in Cali WILL NOT install in a passenger vehicle if they have to do it outside the frame rails. They're just too afraid of liability issues.
Make sure you have that base covered, before you plunk down the money for the tank.