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The King Seeley variable current indicator gauge systems were very reliable but unfortunately, nothing lasts forever. The indicator will function with a variable resistance type sending unit but the indicator wasn't calibrated for variable voltage systems since the Ohms range of the rheostat is just not the same as current based types. Ford, GM, and Chrysler generally used their own designs with different Ohm ranges although there may be a few cross overs. It is possible to match an indicator to a specific range but that requires a change to the design and mounting of the indicator so that it will fit the dash it needs to be installed in and at least looks original. It's basically converting the whole fuel quantity system over to the needed power supply. Most replacement senders are of a universal nature so they can be adjusted to different tank designs.
This link lists Ohm readings for GM, older Ford, and Stewart Warner. There are likely other listings on the net if you search. 36 to 56 Ford replacements are 10-full/100-empty.
https://www.classicinstruments.com/t...sistances.aspx