What happens is this, the pick-up and "mixer" assembly is prone to cracking and allows the pump to suck air at around 1/4 tank. There's probably nothing wrong with the sender, you may actually have a quarter tank remaining when it "runs out".
So, what you want to do is to run it low, use battery power (jumper pack or something) to run the fuel pump with the drain valve open (on the fuel bowl) to empty the rest into cans, and get all the fuel you can from the tank. Then drop the tank, pull the sender/mixer/pick-up unit and inspect it. Then, really weather it's cracked or not, do what's called the Hutch Mod because the whole system sucks anyway!