I have a very similar setup. You will have to cut down both pieces in order to get the rubber fuel hose on.
You can see in the pic, the filler neck only needs to be 1" or so long, same for the "cap" that goes through the bed. You can also put spacers between the tank mounting flanges and the frame to give a little more room. Take the fuel hose and seat it all the way against the tank and secure with a hose clamp. Get it tight! Then slide the second hose clamp (for the cap) onto the hose and tighten it just enough so it will stay where you want it and still allow the cap to go into the hose. Bolt the tank into place and install the cap through the bed wood, into the hose. The two metal pieces should almost touch inside the fuel hose. I have a stepper so this may or may not work on a fleet side but I jacked up the rear end and took the left rear tire off and used a long screwdriver to tighten the second clamp, going through the wheel well area. It's very tight and you almost can't see what you are doing but it can be done. Good luck and keep us posted on your progress.