Fuel gauge not working properly

Just to be sure we are on the same page. When you turn the engine on the fuel gauge goes past the full mark and just stays there the entire time? Then when it is off it goes below the empty mark and obviously stays there?

If this is the case, then I suspect the cluster is the issue.

I tried to find the info on this specific vehicle, but the manual is not detailed at all on older vehicles like this. However, the way these clusters work on newer vehicles is there are two magnets that apply current to them so that it pulls the needle to the proper location based on the input from the sensor.

So, if you have replaced the sending units already, I suspect the cluster is going to need to be replaced assuming you don't have a wiring issue.

So, what I would do is remove the cluster and check voltage as you fill up the tank. If the voltage slowly changes then the sending unit and wiring are fine.

https://www.2carpros.com/articles/how-to-check-wiring

I attached the info below on how to replace the cluster as well.