Just look at this exceptionally clear and concise answer to a question posed by a member of our Classic Mini community on the Mini Mania Forum. I have long thought that our forum is an exceptional resource to get a quick answer to just about all things Mini at just about any time you need it. Members are some of the top experts anywhere in the world…. one in particular is the gentleman who answered this question about fuel gauges and sending units:
“Does anybody know the correct resistance range for an after-market fuel level gauge? This would be installed in a ’65 Mini Saloon with the factory (AHU1029) sending unit in the factory gas tank Several people told me that it was the same as the early GM cars — 0-90 ohms — but when I bought and installed one of those gauges, it reads Full with just 2 gallons of fuel in the tank. So that, obviously, isn’t the correct resistance range. OR, my sending unit is frobbed!”
The answer provided by member “dklawson” (who incidentally has written several articles for us dealing with electrical issues… when he writes, I READ!) gives all the useful info to make the choice you want:
“Minis before late 1964 had the 0-90 Ohm (empty to full) range that you mentioned. That is comprable to some GM guage systems.
From 1965 on the classic Mini had a sending unit with a resistance range of about 270 Ohms = Empty to about 30 Ohms = full. These are nominal values. If you measure a Smiths sending unit it probably won’t match these values exactly.
To use an aftermarket gauge with the bayonet mount sending unit you will need a sender meeting the industry standard range of 240 Ohms = empty to 33 Ohms = full. Your gauge will show empty a bit sooner than true empty but consider that a safety bonus to prevent you from running out of fuel.
Alternatively you can order the fuel gauge wizard from Spiyda Design in the U.K. It will allow you to match almost any gauge with almost any sender.
If you use an aftermarket (non-Smiths) fuel gauge you are not likely to need the voltage stabilizer. Most modern aftermarket gauges have their own voltage stabilizer built inside.”
Here is the whole thread with comments about this topic:
http://www.minimania.com/msgThread/118851/1/1/Fuel_level_gauge_resistance
You will likely see many comments and posts by me (with my real name “Jemal” as my user name). I try to answer questions and maybe provide a little humor and fun and keep the conversation going. Forums like ours are very uncommon! Besides the invaluable and timely answers to questions to help with your project, where else in the world can I say I have so many friends I’ve never met? You are welcome to join me here by posting a question or comment, or on the forum if you want to make yourself a user account by going here.
Classic Mini Coopers 
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