Do you have a close-up picture of the gauge (or gauges)? For example, I'm using a couple of these gauges:-
which requires a sender with resistance ranging from 20 ohms (full) to 260 ohms (empty), like this Triumph TR unit:-
The sender arm can bent bent and shortened to match the tank depth, providing overall travel remains the same. The Smiths gauges also need a 10v input from a Lucas voltage stabiliser or equivalent. They don't work properly on the variable 12 to 14 volts of a vehicle supply.
But you need to start with identifying the gauge, then you'll know the sender resistance range you need. Finally, this might help: