How about put the filler neck where you want it, then get a piece of exhaust tubing with the correct od bent to go from the filler to the tank. The connections between the tubing and the tank, and the tubing and the filler could be made with fuel hose (I get mine at a NAPA store) with the same id as the tubing od. Put clamps on everything, run your vent tube back up to the filler and use a vented cap. Don't try to cheap out and use radiator hose for the connections instead of fuel hose, gas will eat through a radiator hose over time....