I used a 1/4" npt elbow that I picked up at a Auto store. That is the size of the threads on the sender. You will also need a 1/4" to 1/8" reducer to go to your fuel rail as it has 1/8" ntp threads.
Doug
I could not find the 1/8"NPT fittings at any hardware stores - in fact I was told repeatedly that this size does not exist. I ended up getting a NOS part from a speedshop. Don't know the part number - I told the guy that I needed a fitting for a Ford fuel rail and he knew what it was. I know Autometer makes the part too - search their website.