I had to move my gas tank to the rear of my 40 for my roll cage. I want to reclaim my back seat area which is currently taken up by a 20 gal fuel cell. I want to go with like a downey or man-a-fre auxilery tank that mounts under the rear bed pan, but there so expensive. has anyone found a stock gas tank out of like a j**p or something that fits under there with a few mods? or are the aux tank kits worth the money?