Coilover, thanks for the additional information. I will look at the frames in the factory assembly manual but I am guessing that the 3100 and the 3600 are different.
I measured one of my old bed mounting cross rails and it appears to be approx. 1 7/8" high so the filler pipe will need to be 1 3/4" max, probably more like 1 1/2" to have clearance to the bottom of the floor boards. The Tanks Inc 16 or 18 gallon tanks (noted above) have a 1 1/2" ID filler hose so that should be okay if I can get a short enough offset to get the pipe from below the frame top flange to above the top flange.
