Having the tank hang down below the frame rails seems like a really bad idea.  I'll see if I can shoot a picture of mine.  It sits up high, >1" above the bottom of the frame.  That way I can still slide on the frame w/o risking the tank.

There is some space above the tank and in front of the rear axle.  This proved to be a handy place to stuff the air compressor.  Being close to the (rear mounted) battery made for a short wiring path.  

Allowing a switch to the extended range tank as an engine fuel source would be nice.  I didn't design my setup for that, but it can be accomplished in an hour or so by moving around some of the lines.  

Very similar pumps are available for 1/10th that price.  Actual run time will be very low.  Note that a nitrile seal won't last long in biodiesel.  Either way, locating the pump up high, well above the tanks, is important.  You don't want the pump to spring a leak and drool out all your fuel as you drive.

Using the Webasto tap is what I did with my tank. The only real problem with that is that the tap has a small inside diameter, and thus doesn't flow quickly. 

The Front Runner tap has a long tube that goes down into the tank (inside the main fill tube).  See item six on page two of their installation manual.  I thought that was rather clever actually.