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.
Your cross sills are indeed 1-7/8 tall but remember they sit on 2"+ wood blocks making the total space between your wood floor and the frame about 4" in the area where you propose to install the tank. That should make it relatively simple to make the connection over the frame if the tank inlet is near the top of the tank.
Mark
