Well the way I am reading this here, you have

a sender that spans 10 to 73 ohm
a gauge that was designed for a sender that spans 33 to 240 ohm (Roughly 3.3 times what you have)

If you just put a resistor in line (Series), all you are doing is summing the resistance, so you will never get the right reading over the whole range, you will only be able to set it up to be accurate at one specific tank level. What you really need is to multiply resistance. I am drawing a blank on how to resolve this issue, but then I am not a ôlectric magicianö.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs