Another option is not to use the sending unit in the tank and add a flow switch. With this configuration, a relay (contactor) is held closed by the presence of fuel in the flow switch. As soon as the tank empties, the flow switch opens which kills the relay and the pump stops. I've used this setup for over 11 years without any problems. I could probably dig up my circuit layout but it is really simple design. This is the flow switch I used (ES5-21-AC. .25 gpm). It cost about $25 in 2002.
http://www.iprocessmart.com/erecta/erecta_series5.htm#21
http://www.iprocessmart.com/erecta/erecta_series5.htm#21