Sometimes replacement fuel gauge sensors are too long or short for the actual tank depth, put in at a poor angles, or simply get stuck. I have never had a fuel sending unit sensor that I really trusted. I don't know if people really take the time to calibrate them. They tend to pop them in and if they give a reading, they are assumed to be fine
Currently I have an inline Navman/Northstar fuel meter unit that gives me all kinds of info including gallons burned, remaining, low fuel alarm, mpg, gph, fuel consumption curves and assorted other things I don't use. Probably not nearly the quality of a Flo-Scan but it works and is pretty accurate. It can be calibrated and it's also smartcraft compatible should you opt to re-power with a modern fuel-efficient Mercury motor.
The catch is you must always reset your fuel level upon refill as it is just a flow meter with no inside tank sensors...
My actual fuel gauge has a sensor but when over flowing at the vent, it insists that I only have 3/4 of a tank, also because of the quasi-hex-parallelogram shape of the Seafari tank it cant be nearly as accurate as the inline unit which also has the advantage of being situated between fuel/water separator and carb keeping it much cleaner than if in the tank.