I think it's a '52 tank with a '54 sender stuck in it and the fuel line from that sender is capped off.
Exactly. That "vent tube" that's capped is the fuel supply on the '54 sender. Not being used here, since the tank does have the bottom outlet (indicating it's an earlier tank). No need to connect a fuel line to the sender on top.
That thing on the frame rail (pic below) is a solenoid operated selector valve for a two tank setup. It looks like it's been abandoned. It would have a line from each tank and one out to the fuel pump. And some wires to a switch on the dash to change tanks. If the switch is a double pole double throw switch, you could run your sending unit wires thru it so each sending unit would be active when that tank is selected.
Fuel line from tank 1 to one inlet port on the valve, fuel line from tank 2 to the other inlet port, and the outlet port to the fuel pump.
Unless you want to reactivate the two-tank setup (you'd need to clean the switchover valve out and make sure it works on the bench), just pick the tank you want to use and run fuel line from there to the fuel pump inlet.
Your fuel line isn't hooked up. There's an aftermarket fuel filter just before the pump. Maybe you disconnected it to clean out the lines?
Yep, that would make it hard for fuel to get to the carb.
