Big Chevy, the way I was planning to get that to work is with some custom bracketry. I was thinking angle iron between the frame rails to create the upper support, and then straps to wrap around the fuel tank between the angle iron. Stainless steel straps would be my preference. The tank would be situated so that the long filler neck that extends out the back would stick out from under the bed a few inches (below the tailgate) for access. Also, a center access means it doesn't matter what side of the pump you pull up to. That's a feature I love about both my Corvette and my Firebird.
I was thinking about maybe keeping the original tank as a spare. Long trips on the highway, it might be good to have the extra capacity. A simple ball valve could be used with hard line to switch between tanks. I was thinking about having another electric switch to switch between tank pick-ups, so that the fuel gauge would work with either tank.
