Autometer Fuel Guage Problem

I am in the process of building a 1969 Dart ...i have installed a custom Dash with Auto Meter American Muscle Gauges, All gauges work except for the Fuel gauge only reads about an 1/8th of a tank when i know there is at least 1/2 a tank. I tested the gauge with a 10 OHM resistor and it goes to full....I have grounded the tank AND sending unit and taken a ohm reading at the sending unit to ground and it was 39-40.
I called Auto meter Tech support and was told that the new Gauge technology use a non linear signal whereas the sending unit is a linear signal and there is no known fix for this problem...anyone run across this problem?....fix?
just a little more info...new tank ...new OER sending unit...new Painless wiring ....