I know these are old posts but I thought this would help a few people out, as I was in the dark myself and took the gamble.
The factory fuel sender uses a 270-20 ohm resistor. When I installed a fuel cell in my 2004 Rubicon, I used a 240-33 ohm sender from VDO available on Summits website (Summit part number VDO-226007) and my factory gauge is SPOT ON. Attach the hot wire to the post on the top of the sender and the signal wire to the "grounding" post and you'll be in business.
Many are saying you need a 20 ohm resistor according to "the math" for a correct empty reading. However, I believe they are under the impression that this is a 12 volt system. The fuel sender receives a 5 volt signal from the ECM. The 13 Ohm difference at empty is not enough to cause a significant change in the gauges reading. Proper positioning of the sender is critical take your time installing it.