I bench tested a new "quality" repro sending unit on my Duster 340 build. I could never get the new unit to read accurately throughout the fuel gauge scale. I located a NOS sending unit at Carlisle last year. Hooked it up to the fuel gauge and it was spot on. I would recommend trying to track down a NOS unit if possible.

I do not think anyone has mentioned what the OEM spec was. I may have missed it.
Mopar senders were 80 ohms for empty and 10 ohms for full. The aftermarket senders seem to be more in the range of 70 ohms for empty and 20 ohms for full. I think that may be what the Ford ones were.
There is hope:
http://www.technoversions.com/MeterMatchHome.html

I have not installed mine yet but it looks like it should work.
Craig

One other thing to point out: If you have not converted your dash to one of the newer solid state 5V regulators, you will have to use the more expensive unit. It has a built in 5V reference. The old electro-mechanical ones generate electrical hash that disrupt modern electronics.
Craig