I'm not familiar with the B's sending units. However, most Smiths fuel senders are keyed one way or another be they bolt on or bayonet ring mount. I doubt you've put anything in wrong. However, what you may want to investigate is to remove the sender, disconnect the fuel pump and coil, and switch on the ignition. With the circuit powered up this way you can test the response of the gauge to the float arm position. It may be that the range of the gauge you bought is a little out, or it could be that the arm shape requires a little "tweaking" so the float rides at a different position in the tank. Lastly, if your electrical temperature gauge is also reading "wrong" it may be the voltage stabilizer.