I have a 51 Panel & needed all the space previously occupied by the saddle tank for exhaust, trans cooler, etc.
I got a tip from the tech support guy at Jim Carter's a few years ago and acted on it. He said from years of trial and error, he's found that a stock fuel tank for a 71-73 Mustang fits well behind the rear end and between the frame rails. You have to weld up the hole for the fuel filler ( middle of the rear of the tank) , Then cut a new hole for a filler depending on where you want to put it.
I bought a brand new tank from Spectrum for $150 incl. straps. In my case I wanted to recess the filler well into the floor by the back right door. The Mustang straps are not quite long enough to mount under the truck but easy enough to lengthen them a bit. The tank fit perfectly under there and looks like a factory item. Holds a tad over 20 gallons & the AutoMeter sending unit/fuel gauge works as good as any factory fuel gauge set up!
Hope this helps.
