There were two seperate Sending Units used for `65 & `66 applications, and that will be dependant on the engine originally installed in the car. Since you don't have the original engine in your `65, you'll first need find out what the car originally came from the factory with if you don't already know. If you don't, there are many ways to find out even if you don't have any original documentation for the car. `65's could have had anything from an I6 up to a 396/409, and everything in between.
The Fuel Tank Sending Units in `65 will be different for I6 & Small Block V8's vs. those used for Big Block cars. The I6/Small Block Fuel Lines from the Sending Unit were 5/16". The Big Block lines will be 3/8". You indicate that you've gone through two Sending Units in a month, so I'm sure you already know what size the Fuel Line is at the tank, but take a look to ensure that a previous owner has not installed an adaptor anywhere in the line between the tank and Fuel Pump to step up or down in size from the original configuration. Believe it or not, this was a fairly common practice back in the day if someone was doing an engine swap from a Small Block to a Big Block or visa versa. Assuming nothing has been altered here, you'll now be sure of the correct Sending Unit application for the car.
The reason I think you need to check all of these things out first is based on the information you've provided stating that the "gauge only shows 3/4 full when tank is full and at times does not show below 1/4 full when tank is almost empty".
This sounds like there is a strong possibility that the Sending Unit Float is either hanging up in the tank at these fuel levels, or is at it's actual maximum/minimum range of travel. If that's the case, it sounds like these are the wrong units for your application. Secondly, are you using OEM GM Sending Units, or cheap aftermarket assemblies made in God knows where? It would be to your advantage to use actual GM parts here when ever possible. (Just my two cents on that matter).
Once you are certain you have the correct Sending Unit installed, check all of the other obvious and non-invasive things like electrical continuity at every point between the Sending Unit and Fuel Gauge in the car. Use a good quality Fluke meter to do this if possible because you will be looking for accuracy down to the milli-ohm level here, and 43 year old wiring and connectors can get pretty crusty. The wiring schematics for this can be found in the `65 Chassis Service Manual.
If the Sending Unit is working properly, and you have solid electrical continuity throughout the circuit, you should read approximately 240 Ohms at the Fuel Gauge when empty, and approximately 33 Ohms when full. After you're certain that all else is correct, and you aren't seeing these numbers at the gauge, then the gauge itself is likely the culprit.
Hope this helps.
`68 L72 SS427 Impala
`73 SS454 El Camino
`97 Camaro SS Convertible
`07 SC Cobalt SS