The problem is not the A/D converter. The problem starts with the sender itself, and ends at the dash cluster with the gauge, and voltage limiter for the gauges. From one end to the other, this is an inaccuracy waiting to happen.
So the first thing you'd have to do is come up with a quality sender (which I guess no longer exists)
There's lots of "talk" around the www about replacement senders.
On mine, I recently discovered that if it gets down to 1/4 tank on the gauge, you'd better be at the stop light across from the gas pump. I know this because..........