Nick If you are using a new ceramic shunt on the old style fuel gauges you do have a few options.
The easiest is keep the new resistor and to learn where the new landmarks on the gauge are and live with it. But that can be annoying if you forget or someone else is driving and they don't know the new landmarks on the gauge.
But removing the new resistor and getting the correct value is cheaper then buying a new fuel gauge, or looking for an used with the resistor intact. You already have a gauge why buy another. Here are a few option for replacing your ceramic unit.
Option one is to search some on-line sites for an original style NOS or reproduction resistor. I think I last ones I saw offered were at a major Corvette parts house. As with everything beware of NOS offers since an original wire wrapped 40 ohm unit are rare and expensive. The wire reproductions come in many values so double check that it is 40 ohms.
Another option is that you can make your own with resistance wire of the correct length - you would use a meter to find the 40 ohm length, then wrap it around a cardboard, poster board or nylon or plastic "carrier" of the same size and shape as the originals did. The wire can be purchased at any electronics house including Radio Shack.
But baring you wanting it to look factory original for judging then you can buy a standard 40 ohm resistor at Radio Shack and hook it up between the the two leads as the original was and be accurate no arts and crafts skills required.
Another lazy option of using the ceramic resistor is If you have the dash mounted fuel gauge is that you can gently reset the pointer. Open the dash pod face when you have filled the tank. You will reset the pointer to the new full point on the gauge by pulling it out until you feel it disengage, then when in the new location of "FULL" gently push the pointed back in until you feel it click. If you have a console gauge that is not an option as they are not resettable. But I would just replace the new ceramic unit with a RS resistor option and be done.