Ray, thanks for the vote of confidence ! Custom built tanks are worth all of the $1000 or so to do one. If I were making a tank I would be sure to install the baffling foam inside to reduce fire hazards. I have done CAD (cardboard aided designs) designs of some weird tanks for gas and radiator tanks and bent them up for welding but I leave the aluminum welding up to someone who does it all the time for a living.
The guy who's building my tank used to build tanks for the Bayliner boat company. It's currently in "CAD" stage awaiting his workload to die down a bit. It's pretty much going to approximate the standard VW tank, with the exception adding a gallon or so by angling down from the steering shaft to a low point on the passenger side.
As nice as it would be, approximating the 550 tank would double or triple the cost. We're also talking about further increasing the capacity by bring the top of the tank up to the bottom of the lid and eliminating the filler neck, i.e. welding a flange-type cap directly to the top of the tank.
Hes a pretty amazing guy. An old school good old boy that pretty much does everything by eye, and cardboard. The 200 gal tank he made for my brother's Cargyle Cutter is a work of art.