Ford Shoebox - which 12v fuel sending unit?

I have a 1949 Ford sedan. It's got a chevy 350 in it. The person who owned it before me converted it to 12 volt. they also put in a new wiring harness. I don't have the schematics for it. Unfortunately, the only gauge left working is the temperature gauge.

So I've bought some 12volt Stewart Warner gauges and I'm trying to hook them up. I'm starting with the fuel gauge and that is what this post is about.

The gauge is a SW fuel gauge part number 82303. 2-1/16 In (52.40mm diameter) Elec Fuel Level Gauge-240/33 Ohm. 60 degree sweep.

There are two wires going into the back of the existing fuel gauge; one black and one red. The black one is hot.

I suspect, I need a new sending unit because no matter how I hook up the wires to the new unit, as soon as I turn on the ignition, the arrow shoots to EMPTY. And there is a full tank of gas.

My question is, what sending unit should I get? Any advice helpful at this point.