Engine shuts off when fuel tank is at 3/4 tank

More then likely this is a purge valve issue and that it is stuck open. This is not uncommon when these stick open because they are allowing too much fuel vapor from the tank into the engine.

The way this works is the purge opens and allow the intake manifold vacuum to pull vapor off the top of the fuel in the tank rather then allowing it to vent to the atmosphere.

I would suggest changing this out and then retesting this. There clearly could be other causes but this is the most likely cause of stalling or stumbling when related to the amount of fuel in the tank.

I attached the process below on how to replace this or test this if you would like to confirm it.