Cusser
+1y
I'd try a genuine Vise-Grips on those rusted screws, or even cut a slot with a Dremel tool and try a screwdriver. Maybe cut off the screw heads with a Dremel tool, then using PB Blaster (or similar) and Vise-Grips to get the screw part out.
Remember to take precautions for fire danger when working on the gas tank, especially if you end up drilling.
Also, I know you tried a new relay. But I would confirm positive voltage actually getting to the fuel pump, and work my way backward to the relay, etc. if I didn't find positive voltage at the pump.